TWO men have been charged with assault after a bizarre confrontation in Northbridge which started when one of them stepped in front a car and bared his buttocks to the woman driver.

Police said about 7.25pm on November 7 a 39-year-old man and 39-year-old woman were driving along Brisbane Street, Northbridge, when four men stepped onto the road and forced the woman to brake suddenly.

It is alleged one of the men then banged on the vehicle, before pulling down his pants and showing his buttocks.

The group then walked off.

The woman parked the car, before she and her boyfriend followed the group.

She became involved in an argument with the men and was assaulted.

Her boyfriend attempted to intervene, but was then attacked and knocked unconscious.

He received serious facial injuries, including a broken nose, bruises and chipped teeth during the assault.

The four males are then believed to have left the scene.

Police investigated the incident and arrested a 27-year-old Butler man at his home on Thursday.

He has been charged with grievous bodily harm and will appear in Perth Magistrates Court on December 4.

A 21-year-old Butler man was also charged with common assault.

He will appear in court at a later date.