Scaffolding covering a high-rise construction site in the heart of Sydney has caught fire, with dramatic footage showing flames and smoke shooting up the building.

The fire started at the northern end of the Macquarie Street building, near Hyde Park, about 7.50am on Thursday, with fire crews racing to the scene and closing the street.

About 20 people were safely evacuated, a Fire & Rescue spokesman told AAP.

City workers have described the dramatic scenes and the speed at which the flames covered the building.

“The flames were going up the scaffolding. I think there was straw in scaffolding to catch dust which just went up in flames,” Patrick Toohey, who works in a nearby building told AAP.

“You could see balls of hay catching on fire – it was a bit like Grenfell tower going up,” he said, referencing the London tragedy.

Footage from the Nine Network shows flames quickly rising up the 11-storey building, sending a plume of black smoke into the sky.

Fire in Sydney!! #sydney #fire pic.twitter.com/RfuX2SFUa0

— 🤙 Daniel 🤙 (@danielnunchuck) May 30, 2018

Thirteen fire crews and 70 firefighters are on the scene.

“The fire has been confined to the external scaffold; it hasn’t penetrated the internal structure,” a Fire & Rescue spokesman told AAP.

“The fire is under control but not completely extinguished.”

The building is thought to be an older and well-established building that was undergoing a renovation, a Fire & Rescue spokesman told ABC.

NSW Police said there have been no reported injuries.

Macquarie Street remains closed.

-AAP