British authorities say the number of people treated at hospitals after the bombing on the London Underground subway has risen to 29.
Hundreds of British police have embarked on a massive manhunt overnight, racing to find out who placed the homemade bomb that burst into flames aboard a train at the Parsons Green station during the morning rush hour (local time).
The explosion, labelled a terrorist attack by police, ignited a panicked stampede to safety.
The National Health Service says 21 people are being treated and eight others have already been discharged.
The London Ambulance Service says it took 19 patients to hospitals, most with minor injuries. The others went in themselves.
Police say most of those injured by an improvised explosive device on Friday suffered from flash burns.
They say there have been no reports of serious life-threatening injuries.
"Clearly, this was a device that was intended to cause significant harm," Prime Minister Theresa May said after chairing a meeting of the government's COBRA emergency committee.
Witnesses described seeing a "wall of fire" as the bomb, hidden in a plastic bucket inside a supermarket freezer bag, went off about 8:20 am (local time).
The Metropolitan Police force said there had been no arrests so far, but hundreds of detectives, aided by intelligence agents, were looking at surveillance camera footage of the subway, carrying out forensic work and speaking to witnesses.
The site of the blast was in a leafy, affluent part of the city, not near any of London's top tourist sites. British media reported that the bomb included a timer.
Photos taken inside the train show a white plastic bucket inside a foil-lined shopping bag. Flames and what appear to be wires emerge from the top.
Terrorism analyst Magnus Ranstorp of the Swedish Defence University said that, from photos, it appeared the bomb did not fully detonate, as much of the device and its casing remained intact.
"They were really lucky with this one, it could have really become much worse," he said.
Police were first alerted when commuters reported a noise and a flash aboard the District Line train. Commuter Lauren Hubbard was on the train when she heard a loud bang.
"I looked around and this wall of fire was just coming towards us. You just run," said Hubbard, who fled the above-ground station with her boyfriend.
Others described "absolute chaos" as hundreds rushed to flee the danger.
"I ended up squashed on the staircase. People were falling over, people fainting, crying, there were little kids clinging onto the back of me," said Ryan Barnett, 25.
Mark Rowley, head of counterterrorism for the Metropolitan Police, said "this was a detonation of an improvised explosive device."
He said 18 people had been injured, most with "flash burns." Health officials later said four others hurt in the bombing went to the hospital themselves.
Rowley said Britain's domestic intelligence service, MI5, was helping with the investigation, which was being led by the police counter-terrorism unit. He gave no information about potential suspects, saying "It's very much a live investigation."
Witness Chris Wildish told Sky News that he saw "out of the corner of my eye, a massive flash of flames that went up the side of the train," followed by "an acrid chemical smell."
He said many of those on board were schoolchildren, who were knocked around as the crowd surged away from the fireball.
Commuter Richard Aylmer-Hall said he saw several people injured, apparently trampled as they fled.
"I saw crying women. There was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets," he said.
During rush hour, the London Underground train can hold more than 800 people. Trains were suspended along a stretch of the line, and several homes were evacuated as police set up a 50-meter (yard) cordon around the scene while they secured the device.
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