The Brooklyn Museum of Art braced itself today and responded to several days of threats from Mayor Rudolph Giuliani in the clearest way possible: It declared that a controversial show of British art would open on Saturday as planned and filed a federal lawsuit to prevent the city from retaliating.

The suit, drafted by well-known First Amendment attorney Floyd Abrams, seeks an injunction to prevent the city from penalizing the museum by evicting it from its city-owned building or by cutting off $7.2 million in annual city funds. "Once the government starts to fund the arts, it cannot use that power to punish the speech or coerce the silence of those that receive funding," Abrams said in an interview.

A terse statement from the mayor's office, blasting the museum's move as "illegal and ill-advised," said that "the city will end its public subsidy of the museum immediately" and will also "exercise all of its other legal remedies."

A group of artists, museum officials, gallery owners and free-expression advocates plans a rally at the museum on Friday to bolster its resolve. For several days it has appeared that the museum might compromise with the city by agreeing to certain restrictions on the exhibit, "Sensation: Young British Artists From the Saatchi Collection."

Its chairman, meeting quietly with the city's corporation counsel Monday, discussed such measures as removing a collage of a Madonna that incorporated elephant dung, moving certain more provocative works into a separate gallery, and accepting reduced city funding during the exhibit. Giuliani, adding "disgusting" to the roster of adjectives with which he has described the show, indicated that such steps might satisfy the city.

But in a board meeting this afternoon, the museum decided instead to ask a U.S. District Court judge to declare the city's threatened actions unconstitutional. It also voted to drop its policy of not admitting children under 17 to the show unless they are accompanied by an adult, a provision the city claimed put the museum in violation of its lease since it would restrict public access. Warning signs will be posted instead.

While no date for a trial has been set, Abrams said he would be in court as soon as the city, which provides nearly a third of the museum's annual budget, fails to send a monthly check.

"This will be a landmark First Amendment decision," said Norman Siegel, head of the New York Civil Liberties Union, which plans to file an amicus brief. "We commend the Brooklyn Museum for its act of courage in standing up to Mayor Giuliani's bullying tactics."

While New York's other museums have had little to say during the furor over the exhibit, which has already drawn crowds in London and Berlin, John Cardinal O'Connor joined the fray on Sunday. Like the mayor, he found the collaged Madonna an affront to Catholics.

But the city's Democrats, now including likely Senate candidate Hillary Rodham Clinton, have said that while they might find the art objectionable and choose not to attend the exhibit, the mayor was abusing his power.

Rep. Charles Rangel (D-N.Y.), appearing with Clinton yesterday in Harlem, told reporters that "as a former altar boy" he was insulted by the Madonna, the work of artist Chris Ofili. "But I just don't believe my mayor has lived that type of a spiritual life to direct what should happen in our museums."