UK authorities are calling the explosion at Parsons Green Underground station a terrorist attack
The Islamic State (IS) group has claimed responsibility for a blast on Friday that injured at least 29 people on a packed commuter train on the London underground network, the militant group's Amaq news agency said.
The attack at Parsons Green Underground station left some severely burned.
Commissioner Neil Basu, the senior national coordinator for counter-terrorism policing, declared it a terrorist incident, a statement said.
"It is too early to confirm the cause of the fire, which will be subject to the investigation that is now under way by the Met's Counter Terrorism Command."
British police are chasing down the people they suspect of being behind the blast, the country's most senior counter-terrorism officer said.
Mark Rowley said officers were sifting through surveillance footage and examining the remains of the device.
"This is a very complex investigation which is continuing at speed," he told reporters. "We are chasing down suspects."
Britain raised its national security threat level from severe to critical on Friday, meaning an attack is expected.
Prime Minister Theresa May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.
"For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said.
In a pooled interview, which took place after she chaired a meeting of the UK government's Cobra emergency committe, she described the suspected attack as "cowardly". "This was a device intended to cause significant harm."
She added that the UK terrorism threat level will remain at severe, but this will remain under review. There had been media speculation that the threat level would increase to critical because the suspected bomber is still at large.
"The threat level remains at severe. That means that a terrorist attack is highly likely. But this will be kept under review as the investigation progresses. The public should go about their daily lives but remain vigilant. People who are travelling in London will see an increase armed police presence on the transport network and security will be increased," she said.
Police said the blast was caused by as a viable homemade device that appears to have detonated prematurely, amid media reports that a circuit board and a timer had been recovered by specialist officers at the scene.
Passengers were seen "badly burned" following the explosion, witnesses said.
"Explosion on Parsons Green District Line train. Fireball flew down carriage and we just jumped out open door," said Twitter user @Rrigs, who posted pictures of a white bucket smouldering on the train.
The bucket looked like the type used by builders and there appeared to be cables coming out of it.
"We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter.
The station was closed, as well as an entire section of the District Line where it is located and police urged people to stay away from the area.
A Metro.co.uk reporter at the scene was quoted by the paper as saying that some passengers were "really badly burned".
The incident comes after a series of terror attacks that have rocked Britain this year, killing dozens of people and injuring hundreds, putting the capital on high alert.
British Foreign Secretary Boris Johnson appealed for calm in the immediate aftermath of the blast.
"I'm afraid my information is limited and it really is important not to speculate at the moment," he told Sky News.
"Obviously, everybody should keep calm and go about their lives in a normal way, as normal as they possibly can," he said.
Passengers described chaotic scenes at the station in a leafy and normally quiet part of west London.
"There was panic, lots of people shouting, screaming, lots of screaming," Richard Aylmer-Hall, 52, a media technology consultant, told the Press Association.
"There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off," he said, adding that "some people got pushed over and trampled on.
"I saw two women being treated by ambulance crews," he said.
BBC correspondent Riz Lateef, who was on her way to work, said there was "panic as people rushed from the train, hearing what appeared to be an explosion.
"People were left with cuts and grazes from trying to flee the scene. There was lots of panic."
One passenger, named only as Lucas, told BBC Radio 5 live: "I heard a really loud explosion."
"I saw people with minor injuries, burnings to the face, arms, legs, multiple casualties," he said.
Another witness, Sham, told the radio station he had seen a man with blood all over his face.
"There were a lot of people limping and covered in blood," he said.
Emergency services said they were called at about 8.20am local time.
Natasha Wills, assistant director of operations at London Ambulance Service, said: "Our initial priority is to assess the level and nature of injuries."
She said the ambulance service had sent "multiple resources" to the station, including a hazardous area response team.
Trump plays politics
Donald Trump on Friday posted a series of provocative Twitter messages over the explosion in London.
The US president said: "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!" he tweeted.
"Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!" he continued.
Six minutes later, he apparently justified his own domestic policy through what is an unrelated event on the other side of the Atlantic, the motive of which has yet to be determined by British police.
"The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!" he said.
Trump’s online comments brought a terse public response from May, who warned the US president and others not to speculate.
"I never think it's helpful for anybody to speculate on what is an ongoing investigation," May told journalists in London.
Trump later tried to smooth over the rift, describing May as a "wonderful woman."
But his comments were described as "unhelpful" by London's Metropolitan Police, as well as by Nick Timothy, May's former chief of staff.
"True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner," Timothy wrote on Twitter.
After his tweets, and with ties apparently under strain, Trump spoke with May to convey "his sympathies and prayers" for victims of the London attack, the White House said in a statement.
Trump's keenness to underline a series of terror attacks in Britain, virtually in real time, has led to repeated outcry across the Atlantic that has helped indefinitely delay his much-vaunted state visit to the country.