Pictures show queues of motorists and pedestrians forming on both sides.
City of London Police said engineers rushed to fix the bridge. It has reopened to pedestrians but motorists have been urged to find another route.
A witness said they overheard talk of “multiple failures” on security radios.
One social media user said: “I’ve been stuck here for nearly an hour now…” while another wrote: “Yep, Tower Bridge definitely stuck! One side started to come down but the other didn’t!”
The bridge connects the Square Mile financial district to 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 has been approached for comment.