The Met Police evacuated Old Street station on Saturday evening after receiving reports of a man acting suspiciously on a Tube train, with witnesses also describing hearing a "bang".
Officers were called at 9.20pm but found no firearm or other explosive object, the force said in a statement.
The station, a busy interchange close to several nightlife hotspots, was temporarily evacuated, but nothing suspicious was found. No injuries were reported.
Video posted on social media showed more than a dozen emergency service vehicles parked on the Old Street roundabout.
The Met said in a statement: "Police were called at approximately 21:20hrs on Saturday, 30 September to reports of a man believed to be acting suspiciously on an underground train.
"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.
"There were no reported injuries.
"At this stage nothing suspicious has been found and the incident has now been stood down.
"There have been no arrests and enquiries continue."
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