UK raises terror threat level to 'critical'

The terror threat level in Britain has been raised to the highest level of "critical," which means a terror attack is imminent.

The upgrade came following Friday's explosion on a London underground train at the Parsons Green station, West London which injured 22 people.

The U.K.'s five-step threat level has gone up to "critical" -- meaning an attack is expected "imminently" -- according to domestic intelligence agency MI5.

The threat level was last raised to the highest level following the Manchester concert bombing in May, which killed 22, and remained there for several days.

A manhunt is underway for the bomber, who attempted to blow up the tube train during Friday morning rush hour.

Britain's senior counterterrorism officer Mark Rowley told a news conference the explosion was caused by a suspected improvised explosive device (IED).

He said more police would be deployed across London, especially on the capital's transport network.

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In a separate news conference Friday evening, he said a full-scale investigation of the incident was underway, with hundreds of police officers looking at hours of CCTV footage to find out who was behind the attack.

Images from the inside of the passenger train posted online showed a white bucket with wires protruding from it, on fire, inside a supermarket bag.

The Parsons Green station and the roads around it were cordoned off, and police advised people to avoid the area as a manhunt to find the perpetrator continued.

Footage from the scene also showed passengers being led off a train by police.

After the incident 29 people received hospital treatment for mostly flash burns.

With the terror threat level at critical, British military personnel may assist police in protection duties at key sites such as Buckingham Palace, government and parliament buildings, as well as at important events across the country.