Two window cleaners have fallen around twelve storeys after a cable snapped on their platform at the corner of Pitt and Bridge Street in Sydney's CBD.

A witness told 9NEWS one of the men was left hanging by his safety gear after being thrown from the washing cage, while the other lay on an awning after the fall at about 11.15am.

Witnesses said both men, believed to be in their 50s, were hit by pieces of the falling scaffolding.

Paramedics worked to treat the two men. (9NEWS)

The hanging man was later placed on top of a car for treatment.

“The window washer thing came spooling down from about eight or nine floors up,” he said.

“It bounced off the awning.

“The guy who bounced off the awning was left hanging from his safety gear, he didn’t actually hit the ground or the car.”

The scene of the collapse. (Supplied/Steve Ryan)

A NSW Ambulance spokeswoman said one patient had suffered a head injury and a leg injury in the fall.

The other man had suffered a back injury.

Footage from the 9NEWS helicopter showed several ambulances at the scene and the paramedics climbing onto the awning and the car to reach the two men.

Witnesses said dozens of pedestrians rushed to help the injured pair, with the man left dangling by his harness pulled down by bystanders.

They also said there was a “lot of blood” on the fallen scaffolding.

The scaffolding collapsed onto an awning. (9NEWS)

Pitt Street is currently closed between Alfred Street and Bridge Street as a precaution.

Worksafe NSW has been notified.