Holborn station was evacuated this afternoon after armed police searched a train for a man 'acting suspiciously'.
British Transport police say they were called just after 2.35pm, and the station was closed shortly after.
Officers say they were unable to find anyone matching the description of the man and no arrests have been made.
Onlookers tweeted pictures of crowds gathering outside the station with a line of police vans parked nearby. Some commuters said they had been locked inside a train while armed police got on to search the carriages.
The station was shut for around an hour but has now re-opened. However, minor delays are still ongoing on the rest of the line.