UPDATE: BRITISH police say they have arrested an 18-year-old man in connection with the London subway attack.
They say the arrest, in the port area of Dover, is a 'significant' breakthrough in their investigation.
But the terror-threat level, currently at "critical", has not yet been lowered.
The bomber and possible accomplices have been the subject of a major manhunt since the attack wounded 30 yesterday, leaving London on edge.
A Scotland Yard statement says the teen is being detained under the terror act.
"We have made a significant arrest in our investigation this morning," Deputy Assistant Commissioner Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
"The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation. We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place.
"This arrest will lead to more activity from our officers. For strong investigative reasons we will not give any more details on the man we arrested at this stage."
The statement says police have been given 77 images and videos from members of the public, and that 45 witnesses have so far been spoken with.
Police are combing through closed-circuit TV images and have extensively studied the remains of the device without giving details about it. But images from inside the subway car after the blast showed that the device was contained in a bucket with wires hanging out of it and that it was concealed in a plastic shopping bag.
Assistant police commissioner Mark Rowley earlier said that police are "chasing down suspects." The train hit by the bomber had video cameras in each car, and the London Underground network has thousands of cameras at the entrances to station and along the labyrinth of subterranean and aboveground passageways leading from the entryway to the trains.
Officials have hinted there may be more than one person involved, but haven't released details in what is termed an ongoing and covert inquiry.
CHILLING ANNOUNCEMENT AFTER TUBE TERROR ATTACK
A CHILLING announcement on the London Underground after the Parsons Green terror attack has left Londoners horrified at the "new norm".
After a tube driver announced the attack on a Victoria Line train apparently none of the passengers reacted, suggesting that Londoners are becoming desensitised to terror attacks.
The Sun reports Darryl Morris, a presenter and columnist, posted about the incident on Twitter.
He wrote: "Chillingly casual announcement on the Victoria line. 'Delays due to terror incident on the District line'. Nobody flinched. A new normal."
Chillingly casual announcement on the Victoria line.— Darryl Morris (@darrylmorris) September 15, 2017
"Delays due to terror incident on the District line"
Nobody flinched. A new normal.
Many on social media said that this showed the defiant British spirit, one Twitter user commented: "Brits have been this way since the Blitz !"
Another, Sophia Cannon said: "The British literally invented the ability to not panic and carry on.
"We won a war on this rhetoric."
Others reminded Darryl of the IRA attacks which were frequent in the past, writing: "Just like the days of the IRA, bomb practice evacuations, getting on with normal life, keeping eyes peeled. Same old, same old."
However some people said that it was the right reaction, and it wouldn't have been "productive" to panic.
EARLIER: BRITAIN has raised its terrorism threat level to "critical" - meaning an attack may be imminent - after a fiery explosion on a London underground train left 29 people injured.
Prime Minister Theresa May said soldiers will be deployed across London to guard against another attack in the wake of an improvised bucket bomb explosion on a packed commuter train at Parsons Green tube station.
It comes as authorities launch a manhunt for the bomber, who has not yet been identified.
Ms May urged the public not to change their lives but to be alert to the threat.
"The public should go about their business in the normal way but be vigilant and co-operate with police," Mrs May said.
The decision to hike the terror alert came after an earlier announcement that warnings would not change.
The UK was already on a "severe" terror alert, which means an attack is likely. The alert was last increased to "critical" the day after the Manchester bombing in May.
Troops were deployed to guard streets, landmarks and transport hubs across the UK after that announcement.
Soldiers will replace police officers "on guard duties at certain protected sites which are not accessible to the public", Mrs May said.
"The public will see more armed police on the transport network and on our streets, providing extra protection," she said.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
DOZENS INJURED IN TRAIN BLAST
A MASSIVE manhunt was underway in London as police searched for a suspected terrorist who planted a bomb on a peak hour train.
London Metropolitan Police denied earlier reports that a suspect for the attack had been identified using CCTV.
"We have no suspect at the moment," a police spokesman told the Sun.
Sky News earlier reported that British security services had identified a suspect involved in the bombing of the busy commuter train in west London with CCTV footage.
A selection of streets have now been reopened to the public but many remain cordoned off as the investigation continues.
The Friday morning commute in Britain's capital descended into chaos after a "fireball flew down a carriage" at Parsons Green station, in London's southwest.
Police are treating the incident as terror-related, as images emerge of the suspected improvised explosive device.
The blast, which occurred at 8.20am local time (5.20pm AEST) came from a plastic bucket inside a shopping bag, left by the tube carriage door.
Images on social media show several wires coming from the flaming container, which it appears failed to detonate in full.
Reports suggest the device had a timer attached to it.
Panicked commuters fled the train when the doors flung open at the southwest London station in a stampede that crushed people including children as they tried to escape.
NHS England said it had treated 29 patients in relation to the explosion with 21 still in hospital.
All eight patients treated at St Thomas' Hospital following the explosion have since been discharged, a spokesperson for the hospital has confirmed.
All 8 patients treated at St Thomas’ Hospital after today’s incident at Parsons Green have been treated and safely discharged.— Guy's and St Thomas' (@GSTTnhs) September 15, 2017
The Sun quotes an onlooker saying it was like a "fireball flew down a carriage" with panicked commuters diving out an open door.
Passengers said they saw a "flash" and a loud "bang" before the train carriage filled with smoke and the doors flung open.
Irish man Rory Rigney, who was on the train, said he heard a "smash" and screaming before seeing flames.
"I saw the fireball coming towards me - yellow or orange," he said.
"My face still feels warm. I wasn't hanging around to get a better look at it.
"It smelled like a fire extinguisher and there was this foam on the floor. It looked like foam from a fire extinguisher."
Counter-terrorism authorities have taken over the investigation after conducting an initial sweep of the area. Whoever planted the improvised explosive device is still at large.
Hundreds of police were trawling through CCTV footage in an urgent investigation into what appears to be a botched attack.
Security experts suggested it may have exploded sooner than expected because the train, which was headed to central London, was above ground.
Tube passenger Emma Stevie wrote on Twitter that she was forced to "run for my life", saying: "Huge stampede, lots injured. Not sure why. Fire/explosion mentioned."
MAY REBUKES TRUMP FOR TERROR TWEET
UK Prime Minister Theresa May publicly chastised US President Donald Trump after he
appeared to criticise the ability of British police to stop the attack, suggesting the culprits were "in the sights of Scotland Yard".
"Another attack in London by a loser terrorist," Mr Trump tweeted. "These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
Ms May called him out on his comments.
"I never think it is helpful for anybody to speculate on what is an ongoing investigation," she said.
However, Mr Trump argued that he had been briefed on the incident.
"Yes I have, on numerous things happening, including North Korea."
Downing Street confirmed that Mr Trump had called Ms May after his remarks.
"President Trump called the prime minister earlier today to offer his condolences over this morning's cowardly attack in London," a spokesman said.
Ms May convened an emergency meeting of the government's Cobra emergencies committee for 1pm local time as authorities consider whether to lift the official terrorist threat from "severe" to "critical", which would mean another attack was considered imminent.
"My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident," Ms May said in a statement.
'A NUMBER OF CASUALTIES'
London Ambulance Services said 22 people were taken to hospital with a variety of injuries - none thought to be life-threatening - caused by both the blast and the resulting crush of fleeing people.
A local Metro reporter, who was on scene at the time of the incident, claimed several people had sustained serious burns and that "their hair was coming off".
Emma Steveton told BuzzFeed there were "three layers of people underneath me" in a stampede that occurred.
"There was a poor little boy smashed into the floor with his face bleeding. There was a woman shouting that she was pregnant.
"The most traumatic thing was the weight of these bodies piling onto me and just thinking, 'This is it, I'm going to die.'"
Huge police presence at Parsons Green. Was told by a police officer to "get somewhere safe as quickly as I can". pic.twitter.com/944FhPiDzk— Steven Slayford (@stevenslayford) September 15, 2017
A local man, known as Peter, told a local radio station LBC he had suffered minor burns from the explosion.
"Everyone ran off the tube, it was panic stations," Peter said. "I just turned to see and realised it was the one explosion and there wasn't a continuation of the fire."
According to Peter, one man who was closer to the fireball had his a puffer jacket melted off by the flames. There are reports that at least 20 people were injured.
Lucy Steadman, 24, from Brisbane, was caught in a throng of people rushing to escape at Parsons Green.
The Australian was travelling into the city with her partner, Fabian, when the train stopped at Parsons Green.
"We were standing close to the doors of the tube and then I heard some horrible screaming and saw people sprinting down the platform toward the exit," she said. "The station only has one staircase out so people were rushing for that,
"I panicked and ran for that too, thinking Fabian was just behind me but we got separated."
Ms Steadman frantically searched for her partner, but the stampede of frantic commuters had pushed her further away from him.
"At that point, I didn't know what had happened yet and I thought it might be someone with a gun or a knife, so I wasn't sure if the threat was still there or not," she said. "I just had to focus on staying upright because I could see people in front of me falling and getting trampled on".
"When it became clear the immediate threat was gone, we started helping people who had fallen on the stairs," she said.
Ms Steadman used one of the phones she picked up to call its owners mother and "let her know what had happened".
After she found her partner, who had also escaped serious injury, the pair left the station and have been trying to process what happened to them.
"It all happened so fast and I think I'm still shaking," she said.
WITNESSES DESCRIBE 'HORROR' SCENE
Olaniyi Shokunbi, 24, said the horrific scenes at Parsons Green tube station was just like a scene from a movie.
"A woman came out and she just looked absolutely panicked, her legs were all ripped up and she looked like she was burnt," Mr Shokunbi said.
"There was blood all over the place, her hair was all messed up and she was coughing and just shaking, it was crazy".
He also saw a young boy emerge from the mayhem, desperate to locate his family.
"There was a little kid I felt so sorry for him, he came out and he was looking for his younger brother with scratches all over his face. He was screaming and shouting," he said.
"I've never seen such a thing in my life I don't know what to do now it's crazy".
Mr Shokunbi said he was now terrified to catch public transport in London.
"I was just on the phone to a friend saying I don't feel safe taking the train any longer, I don't think you understand," he said.
"If something like this can happen in Parsons Green of all places ... You would think Oxford Street or the other big places where everyone is there but not in Parsons a Green on a Friday morning".
He said he was struggling to comprehend what had just happened.
"What I'd the train was locked and the doors didn't open, I feel like I've been watching a movie".
Daniel Holden was a passenger on the train and took to described the "horror" of the scene.
"Explosion (or) fire on train. People struggling to get out of (the) station," he wrote.
"Whole area closed down - police are pushing people away."
View image on TwitterView image on TwitterView image on Twitter
Another man who said he was on the train, named only as Garth, said on Twitter that the explosion occurred on the rear carriage.
A "mass of people came rushing" after the blast, he said.
Train services through the area have been halted with commuters reporting a "security incident".
Another passenger, Richard Aylmer-Hall, told Sky News UK that the incident sparked "a crush of people".
"Everyone on the tube just ran in panic," he said.
"There was a bit of a stampede going down the stairs. I got up and looked around and there was a lady saying she'd seen a small bag going off."
A man named Peter was injured in the blast and spoke to BBC News from a carpet shop nearby while awaiting treatment.
He said a "fire ball" went over his head and singed his hair, leaving burns to the top of his head.
"It was a really hot intense fireball above my head. There was a lot of people a lot worse than me. I saw a gentleman in a puffer jacket and the whole back of that had been burned. He had burn marks across his face which were a lot worse than mine."
A Reuters witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket. Meanwhile, a passenger named Lucas told BBC Radio he heard the explosion and saw people with "minor injuries".
"Burns to the face, arms, legs, multiple casualties in that way," he said. "People were helping each other."
'COULD'VE KILLED DOZENS'
The bucket bomb was a crudely designed device similar to the Boston Marathon bomb, it's claimed.
Major General Chip Chapman told Sky News: "It's no accident this was both in a confined place and a crowded place. It's more like the Boston bomb. It's almost like a pressure cooker device."
Security experts today told The Sun Online the flaming bucket photographed in the train carriage may have malfunctioned on its way to a larger station.
It appeared to have been constructed out of flammable materials, a plastic bucket and fairy lights.
Security expert Will Geddes told The Sun: "My feeling is that it's a premature detonation. I know both Parsons Green and District Line very well. It's an ideal commuter time.
"It was a premature detonation because it doesn't sit inside the type of profile that terrorists want for a detonation ... Notting Hill or Paddington Station - then you've got a much more identifiable location for the world to pick up on.
"The device is really basic. I don't see a considerable amount of scorched surfaces around the device itself.
"It looks a firework bucket - (the explosion) actually went up and not outwards. I would say it's an ill-conceived device. But there were very significant burns - there was likely to be some kind of toxic element to it."
The first thing Muna Mohamed, 32, did after hearing the news that her usual Parsons Green train had been bombed was call her family to thank them.
"My family are visiting me from Poland at the moment and my five-year-old nephew delayed me slightly this morning so I missed my normal train," Ms Mohamed said.
"I usually get on that train and I usually sit in the first two carriages which is where the explosion happened".
Ms Mohamed had been travelling from Wimbledon to Parsons Green, where she works, when she heard an announcement on her connecting train that there was a security incident on the District Line.
"I'm just completely shocked that I could have been on that train this morning," she said.
"I've lived here for over 24 years and knowing that it could happen at any moment to anyone is just shocking,
"I don't feel safe, this is my home town and I don't feel safe".
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