Image posted on Twitter showing smoke inside a carriage of a Bakerloo line train at Oxford Circus station.

British firefighters said they had extinguished a small fire on an underground train at Oxford Circus station in the centre of London's main shopping district, saying it had been caused by an electrical fault under the train.

"Firefighters dealt with a small fire on a train on the Bakerloo line at Oxford Circus," London Fire Brigade said in a statement. "Four people were treated for smoke inhalation with two taken to hospital as a precaution."

The station reopened following a brief closure, the city's transport authority said.

"The cause of the smoke was an electrical fault under one of the carriages, which resulted in a small fire that was quickly extinguished," said Nigel Holness, London Underground's director of network operations.

"Understandably this was a distressing incident for our customers, for which I give my sincere apologies."

London Fire Brigade posted a picture on Twitter of smoke billowing out of the doors of the train while one commuter shared a photograph of passengers covering their mouths with jumpers and clothing in the train carriage.