The SUV involved in a fiery head-on crash on the Sydney Harbour Bridge was stolen a short time before the accident, police say.

The dramatic crash happened just after 7am on Monday when the north-bound SUV hit an oncoming van, flipped onto its roof and exploded.

The impact bounced the van forward, crashing into a Honda CRV.

The flames leapt metres into the air, casting black smoke over the coathanger in lanes five and six near Circular Quay.

Three people were injured forcing the bridge to close during the busy morning peak period, as bystanders rushed to help trapped drivers.

The accident caused gridlock across the city as the the main arterial route into the CBD from the north was blocked to traffic for around three hours.

Police believe the black Toyota Kluger that flipped after colliding head on with a white Toyota Hiace van was stolen from the CBD shortly before the crash.

Superintendent Rohan Cramsie said the driver of the Kluger, a man in his 30s, was now under police guard at Royal North Shore Hospital as investigators wait to question him, after he suffered serious, non-life threatening injuries.

The owner of the Kluger had been changing his tyre in the city on Monday morning when he was approached by a man under the guise of talking about his car.

“There was a short scuffle … and then the male who was asking about the Kluger got into the Kluger and drove it away, and then headed towards the Harbour Bridge,” Supt Ramsie said.

Dashcam vision has already been posted online, capturing the scene just before, during and after the explosive moment of impact, with Good Samaritans rushing to help the trapped drivers seconds after the crash.

“It’s extremely confronting,” Supt Ramsie said.

“I can only start to begin to imagine what the other drivers would have witnessed and experienced when they saw that vehicle coming towards them which, what we would allege, would be above the speed limit,” he said.

“It’s horrifying to watch.”

NSW Ambulance Inspector Giles Buchanan said numerous bystanders and other emergency service workers risked their lives to help.

“(They) are absolute heroes, we can’t thank them enough,” he said.

“One of the vehicles was well alight when we arrived on the scene, it was an extraordinary effort to get the patient out of the vehicle safely.”

The driver of the van, a man believed to be in his 20s, suffered serious injuries to his pelvis after becoming trapped and was taken to St Vincent’s Hospital in a serious but stable condition.

The third driver, a woman believed to be in her 60s, suffered minor injuries and was taken to Royal Prince Alfred Hospital.

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