A 22-year-old man is in critical condition in hospital after he allegedly jumped onto the back of a white cube van on an east end street early Saturday, Toronto police say.

Witnesses have told police that the man jumped onto the back of the vehicle on O'Connor Drive near Donlands Avenue, according to Const. David Hopkinson, spokesperson for Toronto Police Service.

The man reportedly fell and hit his head, Hopkinson said.

Police and paramedics were called to the area at about 2:15 a.m. The driver of the white cube van did not remain on the scene and police consider the incident to be a hit and run.

"That is what we believe based on the evidence we have collected so far," Hopkinson said on Saturday.

When police arrived on the scene, they found the man unconscious. Paramedics rushed him to a trauma centre in an emergency run, Hopkinson said. The man is now listed in critical condition.

A Toronto police officer stands on O'Connor Drive after an incident early Saturday that police are calling a hit and run. O'Connor Drive, which was closed for hours, has been reopened. (James Morrison-Collalto/CBC)

Hopkinson said the driver may not have been aware of what happened at the rear of his vehicle but the collision is still considered a hit and run because the driver left the scene.

Police would like to speak to the driver and are urging him or her to come forward. Police are also looking for the white cube van.

Officers closed a stretch of O'Connor, from Donlands Avenue to two blocks east of Donlands, until shortly after 8 a.m. to allow police from traffic services to investigate.

Anyone who saw anything in the area is urged to call police.

COLLISION:<br>O'Connor Dr + Donlands Av<br>**2:16 am**<br>- Man jumped onto the back of a moving truck<br>- Fell off + struck his head<br>- Found unconscious<br>- Officers assisting EMS with emergency run<br>- Man in critical condition<br>- Officers looking for driver + white cube van<a href="https://twitter.com/hashtag/GO2497348?src=hash&ref_src=twsrc%5Etfw">#GO2497348</a><br>^dh —@TPSOperations