A cyclist is fighting for his life after he was found by passers-by lying next to his bike on the side of the road with critical head injuries.

The 22-year-old man was found on Grimshaw Street, near Peters Street, in Watsonia in Melbourne's north-east just before 7.30pm.

Police are yet to determine the circumstances around the incident, and whether the man was clipped by a car or has come off his bike.

Paramedics took the 22-year-old to Royal Melbourne Hospital with head injuries.

The hospital confirmed late Wednesday night that his condition had improved from critical to serious.