An underground tube train stands at a platform at Parsons Green underground tube station in west London. Pic/AFP
An underground tube train stands at a platform at Parsons Green underground tube station in west London. Pic/AFP

The UK terror threat was increased from severe to critical as police continue the hunt for the person who attempted to bring carnage to a London rush-hour tube train packed with school children and commuters, officials said on Saturday.

Twenty-nine people including a young boy were injured on Friday when the bomb partially detonated and sent a ball of fire along a carriage of the eastbound District Line train from Wimbledon at the Parsons Green station, south-west London, reports the Guardian. The police ahave found CCTV images that captured the perpetrator as he boarded the train with the bomb packed in a white plastic bucket inside a Lidl supermarket bag.

The train is believed to have had onboard CCTV, and there are a large number of cameras covering the network. The Islamic State (IS) claimed responsibility for the blast, the militant group's Amaq news agency said on Friday evening. There were no reports of life-threatening injuries in the incident which is the fifth terrorist attack in Britain in less than six months.

In a televised statement on Friday night, Prime Minister Theresa May said the UK terror threat level was being raised to its highest rating and 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.

"The public will see more armed police on the transport network and on our streets, providing extra protection. This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses," the Guardian quoted May as saying.

The Metropolitan Police's Assistant Commissioner Mark Rowley said early Saturday that the police were "chasing down suspects" and some 1,000 armed officers would be seen across the country after military assistance was requested. Investigators believe the device was remotely detonated and the bomber may have left the train before the explosion.