British government has raised the threat level in the country from "severe" to "critical," the highest possible, following an explosion on 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.
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.