A huge crane toppled off the new Tappan Zee Bridge under construction and collapsed across the busy span it is replacing, bringing cars to a halt midday Tuesday on the key Hudson River crossing north of New York City.

"Miraculously, there were no serious injuries," said Democratic Gov. Andrew Cuomo, who briefed reporters at the scene. "If you said to me that a boom could fall across the Tappan Zee Bridge at noon, not hit a car... I would not believe it."

Cuomo said the crane was performing routine work on the new bridge, driving pilings into the river bottom, and it was unclear what caused the crane's boom to fall across the entire 90-foot, seven-lane width of the old bridge. Cars swerved to avoid the machinery, and at least one person was loaded into an ambulance, but no one was seriously hurt.

Cuomo said he would not allow traffic back on the bridge until inspectors determine it is safe. It wasn't clear when that inspection would be complete, but Cuomo said it would take at least several hours.

"As you know it's an old bridge," he said. "That's why we're replacing it in the first place."

Nicholas D'Emealio, 21, of Irvington, was in a vehicle about three football fields away from the crane, toward the center of the bridge, when he heard a bang and his driver slammed on the brakes.

"It shook the whole bridge," he said. "At first I thought the bridge was collapsing because this is not a good bridge."

Everybody was OK, but they were stranded. He and his friends got out of their vehicle and threw around a football to pass the time.

The base and treads of the large, movable crane sat on the unfinished new bridge, which crosses the river between Westchester and Rockland counties. After the crash, several boats carrying emergency workers maneuvered in the river around the collapsed crane arm, while other workers appeared to be scanning the water around the wreck.

Rockland County had a helicopter and marine unit at the scene, according to Rockland County Executive Ed Day. The county executive tweeted that the crane operator was shaken but not hurt.

The new Tappan Zee Bridge has been under construction for three years and is expected to be completed by 2018 at a cost of $3.9 billion. It is being built alongside the original Tappan Zee span, which dates to 1955.

In March, a 90-foot tugboat sank after it hit a construction barge near the bridge site, killing three crew members.

In 2013, a powerboat plowed into a construction barge at the bridge, killing a bride-to-be and her fiance's best man. The boat's driver, who had nearly twice the legal limit of alcohol in his system, pleaded guilty to vehicular manslaughter and was sentenced to two years behind bars. The victims' families, however, attributed the crash mainly to bad lighting on the barge.