Ontario's police watchdog is investigating an incident that left a 22-year-old man with a broken collarbone after an officer ordered him to climb down from a tree in a downtown Toronto park.

Sandra Deziel says her son Dylan climbed into the tree during lunchtime a week ago because all of the benches in the small Bellevue Square Park were packed with people enjoying a sunny day.

But she says after he'd sat about 4 1/2 metres up in the tree for a half-hour, a police officer on a bicycle told her son to get down immediately, stating "it is illegal to be in a tree."

Speaking for Dylan, Deziel alleges the officer shoved her son to the ground, breaking his clavicle and leaving him with scratches and a bump on his head before writing him a $365 ticket for climbing a tree without a permit.

She says Dylan did not act aggressively towards the officer, who she alleges rode off on his bike after handing out the ticket.

The Ontario Special Investigations Unit, an independent agency that investigates incidents involving police that result in serious injury, is probing the incident.

Deziel said her son pleaded with the officer that he was no longer breaching the law after he clambered down from the tree.

"His exact words were 'what are you doing I'm down from the tree. I'm not breaking the law anymore,"' she said, alleging he was then thrown to the ground "face-first" and handcuffed as onlookers watched.

"Dylan said to him at least five times — 'my collar bone is broken' — he needed help," Deziel said. She alleges the officer said nothing more and then rode away, leaving Dylan to walk to a nearby hospital.

"His collarbone was broken in half — you should see the X-rays," she added.

Deziel said her son didn't require surgery but is starting physiotherapy next week.