Tower Bridge is stuck open due to a mechanical fault, bringing central London traffic to a standstill.
Photos of the famous bascule bridge show the two arms at different angles, with witnesses saying they failed to come down at the same time.
One Twitter user said: “So have been stood under a raised tower bridge for 15 mins now, hearing rumours it’s stuck.”
A witness told the Standard: “According to the security radios we overheard there are multiple failures.”
Traffic is stationary in both directions and drivers are being urged to use an alternative route.
City of London Police said on Twitter: “Tower Bridge is currently closed to pedestrians and traffic, due a mechanical fault
“Mechanics are currently working hard to fix the bridge. Please find alternative routes.”
Pedestrians said they had been waiting on either side of the bridge for more than an hour.
The two sides are understood to have been lowered by 6pm, but traffic and pedestrians are still being held back.