Ottawa police investigators are looking for witnesses the day after a collision that left a 71-year-old cyclist fighting for his life.

The crash happened on Hawthorne Road, just north of Hunt Club Road, in south Ottawa just after noon on Saturday.

The man had no vital signs when paramedics arrived at the scene, but his pulse was brought back by advanced paramedics on route to hospital. He suffered critical injuries to his head and chest.

The cyclist was wearing a helmet, but the helmet shattered after the man struck the windshield of the car and was thrown "a significant distance", according to Ottawa paramedics.

Two off-duty physicians also stopped at the scene to help. Hawthorne Road was closed between Stevenage and Hunt Club roads for the investigation.