Old Street tube station in London was temporarily evacuated on Saturday night.
The Nothern Line station was evacuated by armed police at 9.20pm.
Police were alerted to the scene when commuters reported that a man had been acting suspiciously on a train.
The Northern Line tweeted: "Old Street station is closed whilst the Police carry out an investigation".
The Metropolitan police said: "Several people subsequently reported hearing a bang. No firearm or any other object was seen.
"Officers, including firearms officers and the British Transport Police, attended the Old Street area and the station was temporarily evacuated.
Old Street station is closed whilst the Police carry out an investigation— Northern line (@northernline) September 30, 2017
"There were no reported injuries."
It's understood that nothing suspicious was found and the incident has now been stood down.
BREAKING: Big incident in London at the moment. Massive police presence. pic.twitter.com/g0PIotBuha— Voice of Europe (@V_of_Europe) September 30, 2017
Police said no arrests have been made and enquires continue.