The British government has raised the threat level in the country from "severe" to "critical," the highest possible, following an explosion at a London underground station.

Prime Minister Theresa May made the announcement on Friday after a briefing at the Joint Terrorism Analysis Centre on the blast that left dozens injured earlier in the day.

The "critical" level of threat means that 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," she added.

Earlier the British police said that the explosion is being treated as a "terrorist incident".

The Metropolitan Police said emergency services were called to Parsons Green station in west London at 8:20am local time (07:20 GMT) following reports of a fire on a commuter train.

A bucket wrapped in an insulated bag caught fire on a packed London subway train, sending commuters stampeding in panic at the height of the morning rush hour.

Photos taken inside a District Line train show a white plastic bucket inside a foil-lined shopping bag. Flames and what appear to be wires emerge from the top.

London ambulance service said they had sent multiple crews to the Parsons Green station, and 18 people were hospitalised, though none had life-threatening injuries.

READ MORE: How the British media covered the Westminster attack

London Transport said services were cut along the line amid reports on social media of the station's evacuation.

Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit, a witness told Reuters news agency.

The Islamic State of Iraq and Levant (ISIL, also known as ISIS), which has said it was behind several attacks on Western cities in recent years, claimed responsibility through its news agency, Amaq.

London "utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life," Mayor Sadiq Khan said.

Several attacks

London has been targeted by attackers several times this year, with vehicle attacks near Parliament, on London Bridge and near a mosque in Finsbury Park in north London.

The London Underground itself has been attacked several times in the past, notably in July 2005, when suicide bombers blew themselves up on three subway trains and a bus, killing 52 people and themselves.

Four more bombers tried a similar attack two weeks later, but their devices failed to explode fully.

Last year Damon Smith, a student, left a knapsack filled with explosives and ball bearings on a London subway train. It failed to explode.