LONDON -- A reported explosion at a train station sent commuters stampeding in panic, injuring several people on Friday at the height of London's morning rush hour, and police said they were investigating it as a terrorist attack.

Photos taken inside a District Line train show a white plastic bucket inside a supermarket shopping bag. Flames and what appear to be wires can be seen. London ambulance services said they had sent multiple crews to the Parsons Green station. Police advised people to avoid the area in southwest London.

A commuter whose train had just stopped there, Richard Aylmer-Hall, said he saw several people injured, apparently trampled as they fled what he described as a packed train. At capacity, the train could hold more than 800 people.

"There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off," he said. "I saw crying women, there was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets."

Aerial footage later showed other commuters being evacuated along the elevated track. The ambulance service said multiple crews had been dispatched to the above-ground subway station.

London's Metropolitan Police said counterterrorism investigators were at the Parsons Green subway station in the southwest of the capital and that "a number" of people were injured in what they described as a fire.

Several people appeared to have burn injuries.

Police say it's "too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met's Counter Terrorism Command."

The underground operator said services have been cut along the line.

British Foreign Secretary Boris Johnson says people should "keep calm and go about their normal live."

London has been struck repeatedly by extremist attacks this year, and the risk of new attack is considered high.

The London Underground has been targeted several times, 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 fully explode.

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

In its recent Inspire magazine, al-Qaida urged supporters to target trains.