Video report by ITV News Correspondent Geraint Vincent
Detectives have identified the terrorist attacker who planted an explosive which went off on a packed tube train, ITV News understands.
A major manhunt is underway as the suspected perpetrator is still on the run.
Twenty-nine people are in hospital, most of whom suffered "flash burns", after the improvised explosive device went off at Parsons Green station in south-west London at around 8.20am.
The so-called Islamic State claimed that it was behind the attack, though it was not immediately possible to verify that claim.
Scotland Yard are now confident that they know the identity of the person who planted the explosive, says ITV News Security Editor Rohit Kachroo.
They are now combing through hours of CCTV and payment receipts to work out further details of the attack.
Photographs of the device shared online showed a burning bucket which had apparently been concealed inside a Lidl bag.
It is also believed to have had a timer attached to it. However, the device appeared not to have detonated fully.
It's thought the attacker dumped the explosive and then fled the scene.
Children were reportedly among the injured, though no one was thought to have suffered life-threatening wounds.
Prime Minister Theresa May has blasted the "cowardly attack".
She said the UK's terror threat would not be raised to the highest "critical" level as it was after the Manchester Arena attack, and remained at "severe".
She also rebuked US President Donald Trump after he suggested that the attacker was someone already "in the sights of Scotland Yard" in a tweet.
Police have not given any indication as to the identity of the attacker, and say they are still carrying out urgent enquiries.
Mrs May said his comments which implied the attack was carried out by people in Scotland Yard's sights were unhelpful.
"I never think it's helpful for anyone to speculate on what is an ongoing investigation," she told reporters.
Emergency services including armed police rushed to the scene and cordoned off the station. Police warned people to avoid the area.
Eyewitnesses reported seeing a "fireball" and a "wall of flame" flying through a packed London Underground train.
Several people were "covered in blood" after a "flash and a bang" and there was a "stampede" of people trying to flee the station.
Lottie Plaschkes, who was on the train, told ITV News: "We pulled in towards Parsons Green station and then suddenly this bag by one of the doors exploded and was on fire."
Sylvain Pennec, a software developer from Southfields, near Wimbledon, was around 10 metres from the source of the explosion when fire filled the carriage.
"I heard a boom and when I looked there were flames all around," he said.
"People started to run but we were lucky to be stopping at Parsons Green as the door started to open."
He described the scene of panic as commuters struggled to escape the carriage, "collapsing and pushing" each other.
Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.
"It looked like a bucket of mayonnaise," he said. "I'm not sure if it was a chemical reaction or something else. I'm not an expert though."
Home Secretary Amber Rudd condemned the blast, saying: "Once more people going about their everyday lives have been targeted in a callous and indiscriminate way. My thoughts are with all those injured and affected."
She paid tribute to the work of the emergency services and urged Londoners "to remain alert and assist the police and emergency services as much as they can".
Mayor of London Sadiq Khan appealed for calm following the terror incident, saying London "will never be intimidated or defeated by terrorism".
The blast comes with the UK on high alert following four terrorist attacks so far this year.
Authorities have foiled 19 plots since the middle of 2013 - including six since the Westminster atrocity in March.
The incident comes a day after figures revealed terror-related arrests in Britain have hit a new record high, with suspects held at a rate of more than one every day.
Police and MI5 are running 500 investigations involving 3,000 individuals at any one time, while there are also 20,000 former "subjects of interest" whose risk must be kept under review.
Anyone who saw anything suspicious should report it to police on 0800 405040