Metronet, a company that maintains the London Underground infrastructure, commonly called the Tube, said the explosion on Thursday morning was caused by a power surge.
Police evacuated several mainline railway and underground stations after a blast near Liverpool Street Station, which serves London's financial district, and a separate incident in the west of the capital.
"We believe there was some sort of explosion. There are some walking wounded at Aldgate," a police spokesman said, referring to an underground station near Liverpool Street.
"We are not sure of the scale of the incident. Reports are still coming in."
A spokesman for London Underground said a second "incident" had taken place at Edgware Road station in west London.
Emergency vehicles rushed to the scene to deal with the confusion.
Witnesses on Sky television were quoted as saying that people streamed out of some of the stations covered in soot, some in tears, many visibly shocked.