Police in London are trying to hunt down a possible network of terrorists after a bomb exploded prematurely on a busy commuter train on Friday morning.

Twenty-two people were injured after the device exploded on a London Underground train as it arrived at Parson Green station in the southwest of the city.

Witnesses heard a bang and saw a wave of flame spread through the carriage.

"There was out of the corner of my eye a massive flash of flames that went up the side of the train," eyewitness Chris Wildish told Sky News, then "an acrid chemical smell".

Many passengers were burned and others rushed off the train in panic, causing more injuries in the stampede. London Ambulance Service said none of the injuries were serious.

London Mayor Sadiq Khan said on Friday that the British capital "will never be intimidated or defeated by terrorism".

He later participated in a government meeting in response to the bomb that was chaired by Prime Minister Theresa May.

The explosion follows two vehicle and knife attacks in London earlier this year which left 13 dead. The four perpetrators, who appeared to be motivated by extremism, were also killed.

Since then, the UK authorities have maintained their terrorism-threat level as severe, meaning an attack was likely.

Images on social media showed the device, a white plastic bucket, with flames coming out of it. Wires protruded from the bucket which was carried in a plastic supermarket bag. The surrounding carriage did not appear to be damaged.

The explosion took place at 8:20 am at the height of the London rush hour, so the carriage would have been packed as it made its way from Wimbledon, on the southernmost end of the line, to central London. The attack took place on a section of the District Line which runs overland in the Fulham area.

Police sources said they believed the bomb only partially detonated. The suspect who carried the bomb may also have been injured. Police were analyzing CCTV footage from the train and the five stations at which the suspect could have boarded it.

AP, AFP and Xinhua contributed to this story.

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