London's Metropolitan Police and ambulance services have confirmed they are at the scene of "an incident" at the Parsons Green Underground station in the southwest of the capital.
London Transport says services have been cut along the line amid reports on social media of the station's evacuation.
A blast at the station on Friday left some passengers with facial burns at the station, London's Metro newspaper reported on its website.
A woman at the underground train station told Reuters news agency that she was injured in a stampede.
"It's rush hour in London and the stations are filled to the brim with people, so it seems like it was a chaotic scene," she said.
"It seems to be some kind of a crude device in a plastic bag that exploded, but emergency services are still evaluating."
Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit, a witness told Reuters.
The man said people were trampled on when they fled the train after hearing a whoosh and seeing flames race towards them.
He said he did not hear any bang reportedly following which police rushed to the Parsons Green station.
"I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," he told Reuters news agency.
He said the train was packed with people.