A blast on a London underground train left some passengers with facial burns, the UK's Metro newspaper reported on its website.

Police said they were aware of an incident at Parsons Green station in West London and that officers are in attendance. London Ambulance said paramedics were present. Heavily armed police are at the scene.
Images posted on Twitter showed a women being attended to by emergency services sitting on a pavement.

London Ambulance Service spokesperson Natasha Will said they received a call at 8.20am (local time) reporting a incident at Parson Green underground station.
"We have sent multiple resources to the scene including single responders in car, ambulance crews, incident response officer and our hazardous area response team, with the first of our medics arriving in under five minutes.

"Our initial priority is to assess the level and nature of injuries. More information will follow when we have it."

A woman on the platform saw what appeared to be an explosion.
"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.

"Some people got pushed over and trampled on, I saw two women being treated by ambulance crews."

He said he did not believe anyone was hurt by the actual device.

A woman at the underground train station told Reuters on Friday she was injured in a stampede.

Reuters reported a witness could see six ambulances, fire engines and a helicopter the station.

Outside the station, a woman was sitting on a pavement with a bandage around her leg while armed police patrolled.

Another witness told Reuters they saw a woman being carried off on a stretcher with her legs covered in a foil blanket.

Police said they were aware of reports on social media and would release facts regarding the incident once they could be sure of their accuracy.

Transport for London said on Twitter that there was no service between Earls Court and Wimbledon on the District Line which runs through Parsons Green.

London's ambulance service said it had sent its hazardous area response team to the scene.

"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team," the London Ambulance Service said on Twitter.

"Our initial priority is to assess the level and nature of injuries."

Police advised people to avoid the area.

"Officers from the Metropolitan Police Service and British Transport Police are in attendance along with the London Fire Brigade and London Ambulance Service," London's police said in a statement.

"We would advise people to avoid the area."

Britain has suffered four attacks blamed on terrorists so far this year which killed 36 people. (Reuters)