Video credit: Milo Avidane

Workers on Monday morning were replacing the water main that burst when a massive heating and air-conditioning unit crashed into Madison Avenue a day earlier, injuring 10 people amid cascading glass and debris.

Madison Avenue is closed to traffic for several blocks and cops are stationed at its intersections with 39th and 38th streets to prevent pedestrians from walking on the block in front of the damaged building.

There is still the possibility of additional debris falling, according to one officer.

A crew of workers was trying to haul the metal unit into one of the control rooms atop the unoccupied office building at 261 Madison Ave. when a crucial “rigging strap” on the crane holding the unit snapped at around 10:45 a.m., said a source familiar with the probe.

“The load on the crane started swinging like it was unstable, and then it crashed into the building. I could just hear all the workers yelling, ‘Look out! Look out!’ ” a witness said.

The unit, which sources said weighed 4 tons, repeatedly crashed into the side of the building as it fell – shattering windows and tearing off metal and concrete along the way.

The huge piece of equipment landed on the avenue between 38th and 39th streets. The block had already been closed to pedestrian and car traffic because of the construction work.

“It sounded like a freight train,” said Milo Avidane, who watched from his apartment across the street as a cloud of dust rose from the wreckage.

“They were rigging the HVAC unit through a hole on the top floor, and it never made it into the building. It landed smack in the middle of Madison, right in front of the restaurant on the bottom floor.”

Zuma, a popular Japanese eatery there, was closed at the time. The building houses offices for companies such as Signature Bank, Aftra and Primedia.

Five motorists and two construction workers were treated at Bellevue Hospital and NYU Langone Medical Center for minor injuries, officials said. Three others also suffered minor injuries but refused medical attention.

“I saw one guy being brought out on a stretcher and a couple cars that looked like they had been damaged,” Avidane said. “It’s amazing no one was killed.”

Mayor de Blasio later held a press conference at the scene.

“Thank goodness this occurred at this hour, on a weekend, when there were not many people around,” he said.

First responders quickly closed several nearby streets out of fear that more pieces of the building’s facade could fall.

The crane appeared to be “in good working order” beforehand, said city Buildings Commissioner Rick Chandler, adding that it recently passed inspection.

He said that the site was granted construction permits in February and that there are currently no open complaints against it.

The crane was being operated by Skylift Contractor Corp., according to DOB records. The machine is owned by a company called Bay Cranes.

This isn’t the first mishap involving the two companies.

In 2010, the license of Skylift operator Christopher Cosban was suspended after he failed to secure a Bay crane that collapsed at a lower-Manhattan construction site in the middle of the night.

Neither company returned phone calls seeking comment.

Additional reporting by C.J. Sullivan