A man has been arrested after three children were knocked down by a car at Crows Nest on Sydney's North Shore.

The crash occurred on the Pacific Highway near the intersection of Shirley Road and Falcon Street about 3.30pm.

Police were told a car had hit three boys, aged between 12 and 14, all of whom suffered leg and head injuries.

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The children were treated at the scene by paramedics before being taken to Westmead Children's Hospital in a stable condition.

Police arrested a man at Lane Cove – several suburbs away from Crows Nest – about 30 minutes later.

A schoolboy in the area at the time told 9News there was "a bit of a bang" followed by a lot of beeping.

"It's a pretty horrible, terrible thing to happen," he said.

Local Dominic Blahut said there were "absolutely crazy" scenes with multiple ambulance and police cars at the scene.

"There were police directing traffic everywhere, ambulances everywhere and police accompanying the ambulances to the hospital," he said.

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Live Traffic NSW said the Pacific Highway is closed southbound at Falcon Street due to the crash.

"Two of three northbound lanes are also closed," the authority said.

"Southbound motorists are diverting left into Falcon Street. Motorists exiting Falcon Street must turn left onto Pacific Highway."

Peak hour traffic around the area is expected to be disrupted for some time.

Local buses are delayed by more than 30 minutes.

Drivers are urged to use an alternative route.

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