“The mayor and the police commissioner have spoken to the need for increasing transparency into the way our city is policed,” Austin Finan, a spokesman for Mr. de Blasio, said. “The release of body camera footage, when possible, is an important extension of that commitment.”

Robert J. Freeman, the executive director of the state Committee on Open Government, who has called for the 50-a statute to be amended or repealed, said the lawsuit was an attempt by the union to further shield police officers from public accountability.

“You have this myth that the disclosure of information relating to the performance of the duties of a public employee in some way relates to that person’s personal privacy,” he said. “Not so. Not so. The record that indicates my salary is about me, but it’s not personal. It’s about me as a public employee.”

The police union, like others in Boston and Seattle, has resisted body cameras for its officers. Officials considered taking legal action against the city during contract negotiations last year until the city agreed to give a raise for officers who would be required to wear them.

While the police union would rather not see the videos released at all, prosecutors prefer that decision to be made after they have completed investigations and decided if criminal charges are warranted. And police-reform advocates have pushed for a standardized process that would allow more videos to be made public.

Darius Charney, the lead lawyer in a 2008 lawsuit that challenged the Police Department’s stop-and-frisk tactics and led to a court-ordered body camera pilot, said the videos should be considered official reports, no different from what officers file each time they conduct a stop, arrest or other enforcement action.

Richard M. Aborn, the president of the Citizens Crime Commission of New York City, a criminal justice policy nonprofit, said that whether the statute applies to body camera video may depend on whether the Police Department plans to use it to evaluate officers’ performances or make disciplinary decisions.