Sydney RBT turns into assault, car chase

A man who tested negative in a random breath test allegedly grabbed a policeman's arm through the window and drove off, dragging the officer along the road for several metres.

A driver who tested negative in a random breath test grabbed a police officer, dragged him along the ground, then sped off prompting a car chase, police allege.

Detective Chief Inspector Garry Sims told AAP a senior constable stopped a 29-year-old man about 9.30pm on Friday in the north-western Sydney suburb of Riverstone.

While the officer was talking to the man, it appeared he was going to drive away, so he reached through the driver's window to turn the ignition off, Insp Sims said.

The man then grabbed the officer's wrist and started driving, dragging the officer on the ground for about eight metres, he said.

Insp Sims said the man kept driving, prompting a car chase involving a helicopter and the dog squad.

Police found the man in a nearby suburb and charged him with a string of offences including assaulting police.

He was refused bail and is expected in a Sydney court shortly.

The officer suffered grazing and abrasions to his arms and hands.