LONDON — A rush-hour fire aboard a subway train on London's Tube network was being investigated as terrorism, police said Friday.

Authorities confirmed commuters had been injured at Parsons Green station in southwest London.

Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counter terrorism policing, confirmed it was being treated as a "terrorist incident."

One photo shared on social media showed a bucket on fire. Wires appeared to be protruding from the supermarket bag it had been carried in.

The Twitter user who posted the image — @rrigs — said he believed the bucket was linked to the incident. He added that an "explosion" occurred and a "fireball flew down carriage and we just jumped out open door."

One video appeared to show the same bucket burning.

Sylvain Pennec, who posted the video, told The Guardian that he "heard a boom and when I looked there were flames all around" just before the train arrived at the station. NBC News could not immediately verify his account of events.

Other Twitter posts showed people fleeing from Parsons Green station, which is located above ground on the District line.

London's Metropolitan Police ordered commuters and residents to avoid the area. A cordon was set up around 500 yards from the station.

The London Ambulance Service confirmed a hazardous area response team was sent to the scene around 8:20 a.m. local time (3:20 a.m ET).

In a statement, it added the "initial priority is to assess the level and nature of injuries."

Commuter Richard Aylmer-Hall told the Associated Press that he witnessed some people getting "pushed over and trampled on" while running away.

Another Twitter user — @Garthserg — said he was on the when a “mass of people came rushing at us after ‘explosion’ on rear carriage. Surreal moment. Adrenaline pumping.”

Daniel Holden, a local politician, posted pictures of emergency services on Twitter.

“Lots of shell-shocked people ” he added.

The London Fire Brigade confirmed it had eight vehicles and around 50 firefighters at the station.

London’s transit system was the target of a series of coordinated suicide bombings on July 7, 2005. Often referred to as “7/7,” the attacks during the morning rush hour killed 52 people and seriously wounded hundreds.

The U.K. has been the target of three deadly terror attacks this year. A total of 33 people have since been killed in vehicle-and-knife rampages on London Bridge and Westminster Bridge in the capital, as well as in a suicide bombing outside an Ariana Grande concert in Manchester.

According to the U.K. government, the current threat from international terrorism is "severe" — the second highest level.