A cyclist is in a critical condition after being slashed multiple times during "a horrendous" armed robbery in North Melbourne, leaving police shocked by the vicious nature of the attack.

The 34-year-old was riding her bike along Harker St at about 2:30am when she was approached by two men and a woman.

They produced a weapon and demanded she hand over her bike and belongings, before attacking her face, arms, neck and chest.

"The victim has suffered some very serious and significant facial injuries as a result of having her face slashed with what appears to be a very sharp weapon, similar to a box cutter," Detective Gerard Whelan said.

"After the attack she's made her way to the corner of Harker and Flemington Road, where she's flagged down a passer-by who's stopped to help her."

He said the woman was left with large wounds and was covered in blood.

Woman 'just going about her business'

Detective Whelan said the victim's injuries were so bad she had undergone surgery for more than six hours and had not been able to speak with police.

"She's going to suffer some significant facial, physical and traumatic injuries," he said.

"My belief at this point is she may have been heading home, she was wearing a bike helmet … doing all the right things and just going about her business.

"It's a random, unprovoked attack.

"I've no doubt she'll have some emotional trauma to deal with.

"I struggle to process in my mind how someone could inflict such horrific injuries on someone, for the sake of just taking a phone or a bike."

Detective Whelan appealed for any witnesses who had seen a group of people of African appearance around the area at the time of the attack.