Traffic congestion has eased in Sydney CBD after commuters were hit with lengthy delays following an accident on the Harbour Bridge.

Three cars collided around 3pm (AEDT) on Thursday near the north pylon of the bridge, causing one of the cars to crash into the pylon.

A 40-year-old woman, two men aged 30 and 50 and a five-month-old baby were trapped in their cars for some time before they were taken to hospital in a stable condition.

The crash sparked traffic chaos in the CBD, with just one northbound lane and two southbound lanes open while emergency services worked to free the victims.

Even after the lanes reopened, the gridlock continued well into peak hour, with huge traffic queues on the southbound lanes stretching back as far as Artarmon at 6.30pm (AEDT).

Traffic in much of the CBD also came to a standstill, with buses leaving Circular Quay running 20 minutes behind schedule.

The NSW Transport Management Centre urged commuters to take trains or ferries wherever possible, with bus tickets valid on those modes of transport until 7pm.

By 7.30pm traffic was once again flowing on the bridge although some buses crossing were still slightly delayed.