A Melbourne woman has undergone hours of surgery and is likely to be scarred for life after thieves slashed her with a box cutter as she was riding home on her bicycle.

The 34-year-old victim was cycling home via North Melbourne when she was stopped by two men and a woman about 2.30am this morning.

The man demanded her bike and belongings before pulling out the box cutter and slashing her face, neck and body, causing critical injuries.

The woman required emergency surgery. (9NEWS)

The thieves ran off with just her mobile phone, leaving the woman bleeding profusely.

The violence of the attack shocked Victoria Police Detective Leading Senior Constable Gerard Whelan.

"I struggle to process in my mind how someone can inflict such injuries to someone for the sake of taking their phone," he said.

The woman was set upon in North Melbourne in the early hours of this morning. (9NEWS)

"I believe it's just a random, opportunistic attack on someone they perceive to be a soft target."

Despite her horrific injuries, the woman managed to ride her bike 50m and walk to Flemington Road to seek help from a passer-by.

The good Samaritan, who called emergency services, was 100m away from the attack and didn't see what happened.

The passer-by told police the victim was bleeding heavily and most of her clothing was stained.

The woman remains in Royal Melbourne Hospital in a serious condition after undergoing hours of emergency surgery.

She's likely to be left with "significant facial injuries, and physical and traumatic injuries".

Police are reviewing CCTV footage to piece together exactly what happened. The three offenders remain on the run.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.