POLICE are hunting a serial sex attacker who preys on young women in Melbourne's CBD in the early hours of the morning.

Police believe the slimly built man, who is described as Asian or South American in appearance, may be responsible for three serious sexual assaults in and around the city since November.

The latest attack was on Saturday morning when the man grabbed a 22-year-old woman by the neck as she was walking in Flinders Lane near Southern Cross Station about 6am.

He forced her to walk past Southern Cross into Collins St and sexually assaulted her in a stairwell.

Police have also linked the man to an incident earlier that night when he followed three young women into the toilets at Southern Cross Station.

The women were able to barricade themselves in a toilet cubicle and the man fled.

After reviewing footage of the two incidents, police have linked the offender to other serious sexual assaults in November and December last year.

On December 7, a 24-year-old woman was sitting on a fence talking on her mobile phone outside a car dealership on Kings Way, near Albert Rd, when the man grabbed her from behind between 2.30am and 3am.

He dragged her into the dealership between several cars and sexually assaulted her, stole her credit card and fled.

The attack came moments after the man had approached the woman as she walked along the street and tried to strike up a conversation. He snuck up behind her shortly after she rebuffed him.

Police released CCTV footage of the man riding a skateboard shortly before the attack.

MORE FOOTAGE OF SKATEBOARD SEX ATTACK SUSPECT

Detectives believe the same man assaulted a 22-year-old woman in an alcove off Bourke St about 12.30am on November 9.

He approached the woman as she was walking west along Bourke Street between Queen and King Sts.