Tasmanian police are investigating how a motorcyclist participating in a charity ride left the road and hit two spectators, hospitalising them both.

Part of a Hobart highway remained closed on Saturday evening as forensic investigators probed the scene of the collision.

Police suspect one of the hospitalised spectators has suffered serious injuries. The motorcyclist was not seriously injured.

Tasmania Police Inspector Jim Semmens said the crash happened just before 3pm, as the annual toy run was winding down.

"The full circumstances of what's contributed to the crash are being investigated," he said.

The event is held by the Motorcycle Riders Association to help the Salvation Army assist needy Australians at Christmas.

A red sports motorcycle was pictured at the scene parked on the footpath, with a motorcycle jacket and helmet in the grass nearby.

Witnesses to the collision are urged to contact Tasmania Police on 131 444.

AAP