Explosion rocked a District Line Underground train in south- west London on Friday morning.
Emergency services are at the scene of the incident.
A picture on social media showed a white bucket inside a supermarket bag, but did not appear to show extensive damage in the carriage.
Witnesses described seeing at least one passenger with facial injuries.
Others have spoken of “panic” as alarmed passengers left the train at Parsons Green station at around 08:20 BST (07:20 GMT).
The explosion is being treated as terrorism, Scotland Yard has said.
Passengers were injured following the blast at 08:20 BST (07:20 GMT) at Parsons Green station in Fulham.
The Metropolitan Police said it was too early to confirm the cause of the fire and the station has been cordoned off.