British police embarked on a wide manhunt on Friday following a terrorist attack on a London subway train.

An improvised device hidden inside a plastic bucket and a supermarket freezer bag exploded inside a packed train carriage, injuring 29 people, mostly with burns.

"Clearly, this was a device that was intended to cause significant harm," Prime Minister Theresa May said after calling a meeting of the government's COBRA emergency committee.

Read more: Cities struggle for security in light of terrorist attacks

Later on Friday, May said the country's threat level had been raised from severe to the highest possible level of critical, which means an attack is expected imminently.

"The public will see more armed police on the transport network and on our streets providing extra protection," May said.

"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."

IS claims link

The so-called "Islamic State" claimed an affiliated group was responsible for the blast, according to the militant group's Amaq propaganda arm. A British counterterrorism police officer said IS often claims responsibility for attacks it has no role in and that authorities were chasing down suspects and leads.

Eyewitnesses told a DW correspondent that some injuries were caused by the blast itself, while others resulted from the stampede that followed as train passengers attempted to rush out of a station normally used by fewer people.

Others described "absolute chaos" as hundreds of people fled the flames.

"I ended up squashed on the staircase. People were falling over, people fainting, crying, there were little kids clinging onto the back of me," 25-year-old Ryan Barnett told The Associated Press.

Investigators scour CCTV footage

British police said hundreds of detectives were making urgent inquiries to find out the identity of the bomber and were backed by intelligence services.

Britain's most senior counterterrorism officer, Mark Rowley, said no one had been arrested in connection with the attack but that detectives were reviewing surveillance camera footage, carrying out forensic work and speaking to witnesses.

The Metropolitan Police force said police were "making fast-time inquiries to establish who was responsible and are working closely with the security services."

"There is a manhunt underway as we speak," London Mayor Sadiq Khan told LBC radio, urging Londoners to remain "calm and vigilant."

"As London has proven again and again, we will never be intimidated or defeated by terrorism," he said.

Device failed

Counterterrorism experts said the device seemed to have malfunctioned, averting a larger catastrophe.

"They were really lucky with this one, it could have really become much worse," said terrorism analyst Magnus Ranstorp of the Swedish Defense University. "It seems that this was hastily put together. Probably not very well mixed together."

Judging from the photos, he said it appeared that the bomb did not fully detonate, as much of the device and its casing remained intact, making it easier for investigators to determine the composition of the bomb.

European leaders sent their condolences to their British neighbors following the incident.

"Our thoughts are of course with the wounded, our thoughts are with the British population," said German Chancellor Angela Merkel, after a meeting with French Prime Minister Edouard Philippe in Berlin.

Philippe said the London attack and a separate knife incident in Paris early Friday "show how much we collectively, in France, Britain and also in Germany, face a major threat."

He added that "we must find answers at the national level and all together ... to give our fellow citizens the greatest possible security," including intelligence cooperation.

Trump's comments unhelpful

US President Donald Trump used the attack to call for stronger action against terrorists.

"These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!" he said on Twitter, without explaining further.

"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 said.

Read more: Does social media help promote extremism and populism?

London's Metropolitan Police labeled Trump's comments as "unhelpful speculation," while Prime Minister May said in response to a question on them: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

Counterterrorism policing chief Rowley said earlier this week that police had thwarted 13 potential attacks in the three years to March 2017. In the 17 weeks since then, authorities thwarted six others but four attacks were successful, not including Friday's attack.

aw/cmk (AP, Reuters, AFP, dpa)