The popular tourist landmark opened earlier on Saturday to allow a ship travelling on the River Thames to proceed through.
But Tower Bridge’s arms did not close as normal, travellers being left frustrated at having to wait for more than an hour to cross.
Long traffic queues have formed at both sides of the bridge and is at a standstill.
Motorists are being warned to expect lengthy delays.
Several people took to Twitter to vent their frustrations.
One wrote: “I’ve been stuck here for nearly an hour now… #TowerBridge.”
A second person tweeted: “Yep, tower bridge definitely stuck!
London’s Tower Bridge is stuck open, causing traffic chaos in the capital
Tower Bridge opened earlier on Saturday to allow a ship travelling the River Thames to pass through
“One side started to come down but the other didn’t! #towerbridge #londontraffic.”
Another wrote: “#TowerBridge opened almost an hour ago and it hasn’t closed back yet.
“It looks stuck and the two parts aren’t symmetrical.”
City of London Police have urged pedestrians and motorists to avoid the area.
Tower Bridge’s arms have failed to close
The force tweeted: “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.”
The Thames RIB (Rigid Inflatable Boats) Experience also tweeted: “Tower Bridge has got stuck.
The technical fault at Tower Bridge has caused traffic chaos
Enginerrs are working to resolve the embarrassing technical fault
“It’s not often that it happens and we’re sure it’ll be fixed soon.”
Tower Bridge has suffered from technical faults in the past.
In 2005, police closed the popular crossing for 20 hours after a technical issues stopped the arms from being fully lowered.
Dozens of frustrated cyclists wait to cross Tower Bridge
Engineers spent several hours trying to bring the arms down and fix the fault.
Each of Tower Bridge’s arms or bascules weighs more than 12 tons.
An additional 400-ton counterweight also helps lower them once river traffic has passed through.