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Britain raised it national threat level to maximum on Friday after a bomb detonated on a London Underground train, with troops set to be deployed in key locations.

At least 29 passengers were injured after an improvised explosive device was partially detonated on a Tube carriage at Parsons Green station in the city's south-west.

Most suffered with flash burns, according to police, while others were injured in the stampede during chaotic scenes at the station.

The terror threat was increased from "severe" to "critical", meaning another attack is expected imminently.

"The public will see more armed police on the transport network and on our streets, providing extra protection", British Prime Minister Theresa May said in a pre-recorded television statement.

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

She said military personnel would take over guard duty at certain closed "protected sites", freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain.

The Islamic State group has claimed responsibility for the explosion, Britain's fifth terror attack in six months.

"The bombing on a metro in London was carried out by a detachment of the Islamic State" group, it said in a statement published by its Amaq propaganda agency.

No-one has yet been arrested over the bombing, but anti-terrorism police chief Mark Rowley said the investigation was making "really good progress".

"We're chasing down suspects," he told reporters.

Earlier on Friday, London Mayor Sadiq Khan said police had launched a "manhunt" following the attack.