LONDON (AFP) - Three London subway stations were evacuated following a number of unspecified "incidents," police said, as witnessses reported panic and screaming in the Underground system.
There were no immediate reports of casualties.
Services on the Victoria and Northern lines were suspended.
"I was in the carriage and we smelt smoke -- it was like something was burning," said Sosiane Mohellavi, 35, was travelling from Oxford Circus to Walthamstow when she was evacuated from a train at Warren Street.
"Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking," she told Britain's domestic Press Association.
"We pulled into Warren Street and were evacuated. It was horrible," she said.
Passengers at Warren Street, one of the affected stations in the north of the British capital, reported seeing smoke but the police were unable to confirm this.
"Emergency services personnel are responding to reports of incidents at three locations on the Underground - the Oval (to the south), Warren Street and Shepherd's Bush (to the west)," said a spokeswoman for Scotland Yard.
The alert came exactly two weeks after the London bombings that killed 52 people and four suspected suicide bombers.
A Sky reporter at the scene of one of the stations reported seeing dozens of fire engines and police vehicles. There were also reports that buildings around Oval station had been evacuated.