The road across Tower Bridge closed on Tuesday night after a man climbed up the structure, prompting a police response.
Traffic and pedestrians were told to find alternative routes due to the incident.
The man has been filmed lying on a metal beam as crowds gather and look on.
City of London Police said: “Tower Bridge is currently closed to traffic and pedestrians due to an ongoing incident. Please find alternative routes.”
The man had been up on the steel wires for at least one hour and it has been reported on social media that traffic is backing up through the centre.
Police boats also arrived on the scene.
One witness told the Standard: “Police, ambulance, [and the] fire brigade [are] all on the middle of the bridge with officers on the pavement situated below him.”
