The officer was in stable condition after the shooting, which happened early on Saturday outside the 25th Precinct station house.

A New York City police officer was hospitalized in stable condition after being shot while sleeping in a car outside a station house in East Harlem early on Saturday, Mayor Eric Adams said at a news conference hours after he was sworn in as the city’s leader.

The officer, whose name was not released, had worked an eight-hour shift on New Year’s Eve, officials said, and went to sleep in his personal car in a parking lot next to the 25th Precinct station house on East 119th Street to rest before his next shift began at 7 a.m.

When the officer woke up around 6:15 a.m., he noticed that his rear window was shattered and he felt pain in his head, officials said.

The police said they determined that the bullet was fired from a distance. There was no initial indication that the officer had been targeted, and no suspect had yet been identified. Investigators will determine if the shot was a stray bullet, officials said.

But Mr. Adams, who visited the officer and his family at NewYork-Presbyterian Hospital/Weill Cornell Medical Center in Manhattan, said the shooting was reflective of a problem that needed an immediate fix.

“This is a horrendous act that took place,” Mr. Adams said. “We don’t start bringing in the new year with bringing in violence. It’s unacceptable.”

Keechant Sewell, the new police commissioner, said at the news conference that no officers who were outside the station house at the time heard shots fired and that there had not been 911 calls in the area for gunfire. The shooting was also not caught on video cameras at the police station, she said.

When the officer, who Commissioner Sewell said is a seven-year veteran of the department, exited his vehicle, others noticed that he had blood coming from his head. He was taken to the hospital, where he underwent surgery and doctors determined that he had a fractured skull, Commissioner Sewell said.

“We are lucky, fortunate and grateful,” she said. “This incident underscores that there are too many guns out there in the wrong hands.”

The shooting on Saturday served as a stark example of what Mr. Adams has made a central part of his campaign platform as he enters office.

Gun violence had reached record lows in New York in 2018 and 2019. But during the coronavirus pandemic, shootings jumped significantly — remaining far lower than the peaks of the 1980s and 1990s but inflicting a major toll on some neighborhoods, including the section of East Harlem where the officer was shot on Saturday.

Gun violence experts said the trends had shown signs of improvement and had started to level off in recent months, but the numbers remained higher than prepandemic levels.

The rest of Mr. Adams’s first day in office struck similar themes about fighting crime.

Earlier in the day, he was at a subway station in Brooklyn when he called 911 after seeing a fight break out on a nearby street. A police car eventually arrived after the situation had calmed down, and officers did not intervene.