About 15,000 nurses walked off the job Monday at three major private hospital groups in New York City, protesting pay and working conditions.

Officials declared a state of emergency as the work stoppage unfolded. The New York State Nurses Association (NYSNA) said on its website that the strike came after months of stalled contract negotiations and is the largest in the city’s history.

Picket lines were set up at hospitals including New York-Presbyterian, Montefiore Bronx, and Mount Sinai.

"Unfortunately, greedy hospital executives have decided to put profits above safe patient care and force nurses out on strike when we would rather be at the bedsides of our patients," said Nancy Hagans, NYSNA’s president. "Hospital management refuses to address our most important issues -- patient and nurse safety."

New York Mayor Zohran Mamdani, a Democratic socialist, joined the nurses in support. "We know that during 9/11 it was nurses that tended to the wounded," he said. "We know that during the global pandemic, it was nurses that came into work, even at the expense of their own health." Wearing a red NYSNA scarf, he urged both sides to "return immediately to the negotiating table and not leave. They must bargain in good faith."

The strike forced hospitals to discharge or transfer some patients, cancel procedures, and bring in temporary staff.

A Mount Sinai spokesperson told CBS News: "Unfortunately, NYSNA decided to move forward with its strike while refusing to move on from its extreme economic demands, which we cannot agree to, but we are ready with 1,400 qualified and specialized nurses -- and prepared to continue to provide safe patient care for as long as this strike lasts."