A DARWIN man got the fright of his life when his car broke down in Canberra and a bogus copper savagely attacked - but real police swooped within minutes and capsicum-sprayed his tormentor on the bum.

Jeremy Marquez, 20, from Lyons drove his shiny Subaru Impreza WRX from Darwin to Canberra where he has moved to study IT at university.

He had only been there a week when he was attacked after driving some friends home late at night.

"The car alarm malfunctioned and shut down the car," he said.

Mr Marquez called the NRMA but almost as soon as he did a random stranger appeared at his window, claiming to be a police officer.

"He said 'get out of the car and hand me your keys and registration papers'," he said. The man started beating the window, trying to smash it with his fist.

Mr Marquez said that the man didn't have a police uniform on, just a T-shirt and denim shorts.

"He started banging on the window straight away," he said.

"It was close to breaking, I thought it would break but it didn't."

Terrified, Mr Mar- quez called the police for help and then told the attacker he'd give him two minutes to run away, and showed him the phone.

But the powerfully-built man kept pounding on the window. Minutes later three police cars swooped and despite being outnumbered seven to one, the man turned to attack them.

"He ran at the cops and tackled one in the air, and it took two other cops to restrain him," he said.

Mr Marquez said police sprayed his face with capsicum spray but the man kept on attacking them with his eyes closed.

As he tackled the officer his shorts came half off.

"So they ended up spraying on his bum," he said.

"They sprayed his anus."

"They just sprayed it incidentally ... and it stopped him."

Mr Marquez said the capsicum spray on the man's naked bottom seemed to have more effect than when they had sprayed it on the man's face.