NEW YORK — The city cannot prevent a group planning to demonstrate outside City Hall from using a sound amplification system because it would violate the First Amendment, a federal judge ruled yesterday.

The ruling by U.S. District Judge Kimba Wood frees organizers to use microphones at Dec. 1 demonstrations criticizing the city's AIDS policies and other activities marking World AIDS Day.

Housing Works Inc., which sponsors the activities, had sought in a lawsuit filed by the New York Civil Liberties Union to obtain the use of sound in the plaza immediately outside City Hall.

Wood rejected arguments by city lawyers that a ban on most amplified sound in the plaza is necessary to avoid excessive noise that would disrupt the business of government.

She said the ban violates the First Amendment requirement that regulation of speech not rely on the content of the speech in question.

"The exceptions that the city is permitted to make vest too much discretion in city officials," she wrote.

She noted that the city allows sound amplification for ticker-tape parades honoring aviators and astronauts, athletes, soldiers and visiting dignitaries and statesmen. She said the discretion of city officials allows the city to support or suppress a particular point of view.

"A sound amplification ban cannot stand when it vests officials with this type of discretion," she said.

A city lawyer did not immediately return a telephone message for comment.

Chris Dunn, a lawyer for the New York Civil Liberties Union, said the ruling was another in a series of federal court victories against a city administration that was seeking to silence viewpoints it dislikes.