A manhunt has been launched after a bomb was detonated in a suspected terror attack on a packed London Underground train.
Witnesses described scenes of terror and panic as the blast sent a "fireball" and a "wall of flame" through a District line service at Parsons Green station in West London.
Police suspect the explosion was sparked when an improvised explosive device was detonated, and have launched a huge counter-terrorism investigation alongside MI5.
The device is being forensically examined while detectives are scouring CCTV and examining witness statements for potential leads.
London Mayor Sadiq Khan told LBC radio "there is a manhunt under way as we speak", while Scotland Yard said detectives are making "fast-time" inquiries to establish who was responsible, adding that there have been no arrests.
As Britain faced up to another terrorist incident following four attacks already this year:
* Images emerged on social media appearing to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage;
* Reports suggested the device had a timer;
* A total of 22 patients were being treated in hospital, with a number of those hurt suffering from burns;
* Scotland Yard declined to comment after US President Donald Trump tweeted: "Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
* Home Secretary Amber Rudd said: "Once more people going about their everyday lives have been targeted in a callous and indiscriminate way."
* Prime Minister Theresa May called a meeting of the Government's Cobra emergencies committee.
Emergency services including armed police rushed to the scene and cordoned off the station.
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, but it looked home-made. I'm not an expert though."
Chris Wildish, who was on the train, said he saw a "device" in the last carriage.
He told BBC 5 live: "Flames were still coming out of it when I saw it and had a lot of wires hanging out of it - I can only assume it was done on purpose.
"It was standing against the door of the rear-most carriage."