A heating and air conditioning unit hoisted 30 stories above a Midtown high-rise crashed to the ground on Sunday morning, injuring 10 people and leaving plumes of smoke in its wake.

Officials said the accident happened around 10:43 a.m.  at 261 Madison Avenue between 38th and 39th Street, when the crane dropped the unit, which hit the building several times before crashing into the street.   

The impact sent debris flying, injuring 10 people. 

“Thank goodness this occurs at this hour, on a weekend, when there were not many people around,” Mayor Bill de Blasio said at a press conference after he surveyed the damage.

Two construction workers were taken to Bellevue Hospital Center, and eight others were taken to NYU Langone Medical Center, said Assistant FDNY Chief Ronald Spadafora. None of the injuries are life-threatening, and EMS chief James Booth said everyone was awake.

Eyewitness Ian Duncan Ball said he was going into a friend’s building near the scene when the unit fell.

“I was standing next to the doorman and we both kind of looked at each other and said they better not drop that thing,” Ball said.

“Imagine throwing a bus off of a building,” Ball said, estimating the unit weighed a few tons and was the size of a school bus. “It was a hell of a crash … the way it was situated on this crane, it was way too big for the height it was at.”

Traffic was blocked off between 34th and 42nd Streets. The mayor said he hopes all roads will be open by rush hour on Monday morning.

“We didn’t hear anything, but we were playing loud music,” said a barista working at a nearby Starbucks on Madison Avenue at 41st Street.

The building, which is not far from Grand Central, is described as a 28 story, 385,046 square foot office tower. It is owned by the Sapir Organization. The crane was operated by Bay Crane. The organizations did not return calls for comment.

Department of Buildings Commissioner Rick Chandler said the crane was in good working order and construction at 261 Madison Ave. started in February. Officials said the city will conduct a thorough investigation.