A massive crane collapsed on the Tappan Zee Bridge Tuesday, crushing part of the aging span, spewing chunks of metal across it and injuring four people, authorities said.

Miraculously, no cars were struck as the crane’s huge red arm smashed down across the bridge’s seven lanes at about noon, halting vehicles in both ­directions, authorities said.

“The crane came down right in front of me,” shaken driver Balam Arana told The Post. “I felt lucky, for sure. It could have been me crushed under the crane by seconds.”

A construction worker and three people involved in two fender-benders were treated for “very minor injuries” on the bridge, which connects Tarrytown in Westchester County with Nyack in Rockland County.

The crane operator was briefly trapped in his cab but rescued and uninjured, officials said.

“If you said to me a boom could fall across the Tappan Zee Bridge at noon, not hit a car, not hit a person and there were no serious injuries, I wouldn’t believe it,” Gov. Cuomo told reporters in Tarrytown after cutting short an event in the Finger Lakes to fly by helicopter to the scene.

Cuomo said the crane was one of 28 working on a new bridge being built next to the Tappan Zee and “operating with what they call a vibratory hammer.

“It was a new crane performing a routine task. This was not one of the high-risk operations of bridge building,’’ Cuomo said.

A structural inspection was performed on the damaged span before it was partially reopened around 5:15 p.m. All but one southbound lane from Rockland to Westchester was expected to be open by Wednesday morning.

“At least one lane on the south side appears to have had significant damage,’’ Cuomo said, referring to a section of concrete barrier that had been taken out by the tip of the crane.

Terry Towle, president of Tappan Zee Constructors, the prime contractor on the new bridge, said wind was not a factor in the accident.

He said the crane was being used to hold the 121,000-pound hammer, which was driving in piles, or long steel poles, for the new bridge’s foundation.

The crane collapse was only the latest mishap tied to the $3.9 billion replacement of the Hudson River span, which was built in 1955.

On March 12, a tugboat smashed into a construction barge and sank, drowning the three crew members. On July 26, 2013, a drunken boater plowed his craft into a construction barge, killing two passengers and injuring three others.

The new bridge is set to open in 2018.

Additional reporting by Bruce Golding, Kevin Sheehan and Kate Sheehy