Police have launched a manhunt after a London Tube passenger was reportedly robbed before being sprayed in the face with an unknown substance.

British Transport Police said officers attended Elephant and Castle Tube station, in southeast London, at just after 9.30pm on Wednesday morning to reports of a robbery on board a train.

"The victim, a man, reported he had been sprayed with a substance. He was treated at the scene by paramedics; the victim suffered a minor irritation to his face," a spokesperson said.

Officers remained at the scene, the spokesperson added, but said test had confirmed the substance was not acid.

Footage shared on social media showed police cars, an ambulance and a fire engine outside the station on the Elephant and Castle roundabout.