A cyclist was left fighting for her life Tuesday following a morning rush-hour collision with a garbage truck that initially left, but then returned to the scene of the downtown crash.

The twisted bicycle of the woman, 38, lay for hours in the taped-off Yonge Street intersection as police reconstruction experts examined the scene.

The incident occurred at about 8:45 a.m. near Ryerson University as the woman rode across Yonge Street at Gerrard Street.

The "Green For Life" garbage truck, travelling in the same direction, made a right turn, its back wheel crushing the woman, police said.

"Apparently, the vehicle did leave the scene but has returned," Const. Clint Stibbe said.

Police would not comment on a report that a witness had chased down the truck in a taxi.

It was still too early to determine who might have been responsible but Stibbe said the driver, 37, was co-operating with the investigation.

No arrests had been made and it was premature to speculate about any charges, Stibbe said.

Police were unable to offer any details on the victim beyond her age but said she was taken to hospital "suffering serious life-threatening injuries."

They appealed for witnesses and were looking for any store surveillance video that might shed light on the incident.

Collision experts crawled under the green garbage truck, which showed clear signs of front-end damage, as they looked for clues to the mishap.

The impact of the crash tore the saddle of the mangled bicycle. Police cordoned off the scene with yellow tape, blocking the city's central thoroughfare.

The crash occurred amid an ongoing debate about cyclist safety in the city and a dispute over bicycle lanes on Jarvis Street, a few blocks over to the east. City council has agreed to remove bicycle lanes over the objections of cyclists, who had long fought to have them installed.

A protest on Monday delayed the removal of those lanes.