Waterloo station was briefly evacuated during Friday morning's rush hour after a passenger alarm on a platform was accidentally set off.
A spokesperson for British Transport Police (BTP) said the alert sounded at 9.01am from the Underground station, which links the Northern, Bakerloo, Jubilee and Waterloo and City lines.
Passengers unaware the evacuation was caused by a false alarm spoke of their shock on Twitter as signs outside the station displayed red warnings reading: "Emergency, do not enter".
Stephen Morrison wrote on Twitter: "Being evacuated from a tube station will definitely wake you up in the morning. Slightly scary start to the day."
A spokesperson for BTP said the station was reopened by 9.15am and movement was returning to normal.
"A platform alarm on the Tube was activated by mistake but it has been re-set and the station is being reopened," she added.
Waterloo is London's busiest station, seeing more than 89 million passengers arrive and depart every year, according to Transport for London's latest figures.
Nearby Charing Cross station was closed half an hour later after also being evacuated when an unattended bag sparked a security alert. It was found to be safe and the station re-opened within minutes.
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