Police have surrounded several streets at London Bridge to stop people entering an area occupied by Extinction Rebellion protesters.
Pedestrians coming from London Bridge Underground station are being diverted away from the protest by dozens of officers and police vans on one road alone.
Demonstrators parked a bus in the middle of a road near London Bridge as part of the protest.
Activists have attached themselves to the vehicle as well as the road.
An officer confirmed some people taking part in the protest were being arrested, and the road closures would be in place "for hours".
Scotland Yard said in a statement: "Officers are responding to a demonstration in London Bridge where protesters have assembled and are blocking the road.
"We are currently assessing the situation working to keep disruption in the area to a minimum.
"Officers are on scene actively engaging with those assembled at London Bridge. Specially trained officers are working to safely remove those who have attached themselves to the vehicle and the road."
Extinction Rebellion London Bridge protest, in pictures
Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.