Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017. (AFP/Daniel Leal-Olivas)

A witness said the fire flared up from a carriage and then spread across the train. The incident made passengers stampede from the scene. Another commuter said she saw a bag, a light, and a blast.

According to London Fire Department, 6 fire trucks and 2 units of 50 firefighters have been sent to Parsons Green station. Anti-terrorist forces are closely overseeing the area.

London police said they had been informed of the incident. There were reports on the injured. Images on social media show that some people have been under medical care.

The blast occured at 8.20 a.m. local time - during the city's rush hour when the underground system was crowded. Parsons Green station has been closed until further notice.

The same day, the British Police confirmed Parsons Green Tube explosion was a terrorist attack.

Prime Minister Theresa May extended her sympathy to the victims and said she will chair an emergency meeting later on.