Tower Bridge was open for more than an hour, leaving hundreds of people and cars stranded in central London.
The famous crossing failed to close after allowing a ship to cross the River Thames on Saturday afternoon.
Images show queues of motorists and pedestrians forming on both sides.
London City Police said engineers were in a hurry to fix the bridge. It has reopened to pedestrians, but motorists have been urged to find another route.
A witness said they heard talk of “multiple failures” on security radios.
One social media user said, “I’ve been stuck here for almost an hour now …”, while another wrote, “Yep, Tower Bridge stood sure! One side started to fall, but the other didn’t!”
The bridge connects the Square Mile financial district with Southwark.
In 2005, police closed the bridge for 10 hours after a technical problem meant the arms could not be lowered.
A Tower Bridge spokesman was approached for comment.