Armed police stormed two Tube stations at the height of rush hour over reports of a man wielding a knife.
Passengers were left terrified as British Transport Police (BTP) put Bank and Liverpool Street stations in lockdown as they searched for the armed man.
More than 20 armed police attended the incident and boarded trains according to passengers, but no suspect has been found.
Police were called shortly after 5pm local time.
A BTP spokesperson said: "BTP were made aware at 5.11pm of reports of a man with a knife at Bank Station. Both Bank and Liverpool Street stations were searched by officers with no trace of the male and enquiries are ongoing. Trains were stopped as a result."
Passengers report that Bank Station was put in lockdown following the incident and Transport for London (TfL) suspended services on the Central line between Leytonstone and White City for almost an hour.
Commuters on Twitter said armed police were at the station. They also described how people were screaming as officers told them to move.
Trains later began running again but with severe delays, Transport for London said.
British Transport Police said officers searched both Bank station and nearby Liverpool Street station but found "no trace of the man".