Three elderly people have been hospitalised after a head-on crash involving two vehicles on Sydney’s iconic Harbour Bridge.

Emergency services were called to the bridge at 2.12pm on Tuesday following reports of the crash.

NSW Ambulance said four people were involved in the crash, with one person declining assessment.

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A 72-year-old man suffered a chest injury and was taken to St Vincent’s Hospital while a 76-year-old woman was taken to the same hospital with a spinal injury.

An 88-year-old woman was taken to Royal North Shore Hospital with a chest injury.

All three patients were in a stable condition.

Two vehicles crashed head-on. Credit: 7NEWS

NSW Ambulance said four people were involved in the crash. Credit: 7NEWS

Helicopter footage captured by 7NEWS shows both vehicles with extensive front-end damage, surrounded by a large emergency response.

The crash, on the southern side of the bridge, closed three northbound lanes, before they reopened later.

However, traffic heading towards the Harbour Bridge remained heavy from the Anzac Bridge, as well as on the Cahill Expressway over Circular Quay and through the Domain Tunnel.

The Transport Management Centre said some outbound bus services were delayed by up to 20 minutes.

Motorists and bus passengers are advised to allow plenty of extra travel time until traffic conditions returned to normal.