Old Street station was evacuated on Saturday evening (30 September) following reports of a man acting suspiciously.
Eyewitnesses reported hearing a loud bang, sparking an armed police response as 20 ambulances rushed to the scene in East London.
One eye-witness described the scene tweeting: "Just been evacuated out of Old Street station. Armed police came to the platform and told us all to leave immediately."
A Metropolitan Police spokesman later confirmed the incident was a false alarm.
"We 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."
British Transport Police also attended the scene, tweeting: "We attended Old Street station following reports of a security alert. A search has taken place to ensure the area is safe. Station reopening."
London has been on high alert following a series of terror attacks on the capital. On 15 September, 30 passengers were injured after a home-made bomb was left on a District Line train at Parsons Green station.
Police have made seven arrests in connection with the terror attack.