(NEW YORK, NY) -- Cardinal John O'Connor is siding with the mayor in attacking a museum's plans to exhibit a portrait of the Virgin Mary that features a clump of elephant dung and pictures of buttocks.

But civil rights activists say the mayor is violating the First Amendment by pledging to withhold $7 million in city funds from the Brooklyn Museum of Art. The show "Sensation: Young British Artists From the Saatchi Collection," opens Saturday.

"I'm saddened by what appears to be an attack not only on our blessed mother ... but one must ask if it is not an attack on religion itself and in a special way on the Catholic Church," O'Connor said Sunday in his weekly sermon at St. Patrick's Cathedral.

O'Connor did not name Mayor Rudolph Giuliani, but he said he was grateful to city officials, adding: "It is their right, if not their duty, to express themselves on such matters."

He urged his listeners to send protest letters to the museum. But New York Civil Liberties Union director Norman Siegel said the threat to cut funding violates the First Amendment.

"His assertion that New York City can withdraw all funds for the museum based on a single exhibition that he finds offensive illustrates a serious misunderstanding of the Constitution," Siegel said.

Siegel is organizing a Friday rally in support of the exhibit outside the museum.

The museum's director, Arnold Lehman, has not publicly said what he will do, but he has a reputation for standing firm on matters of artistic expression. Directors of other museums have been noticeably silent.

"There's a chill in the air because people are afraid of the mayor, but if he wins this one there are huge consequences to artistic expression," Siegel said.

The painting, "The Holy Virgin Mary," depicts Mary with dark skin, African features and flowing robes. It features a shellacked clump of elephant dung and two dozen cutouts of buttocks from pornographic magazines.

The artist, Chris Ofili, 30, last year won Britain's prestigious Turner Prize, a $33,000 award for artists under 50.

Ofili, a Roman Catholic, was quoted in The New York Times as saying that he used the pornographic images because classical images of Mary are often "sexually charged."

He began using elephant dung during a six-week stay in Zimbabwe. He is now famous for using the material - which he gets from the London Zoo - in virtually all his artwork and calls it "quite a beautiful object."

Messages left at his London residence seeking comment on the controversy were not immediately returned.

The same show caused a stir two years ago when it was displayed at the Royal Academy of Art in London. Three academy members resigned in protest when it opened, but record crowds came to look.

Among its other provocative pieces are Damien Hirst's "This Little Piggy Went to Market, This Little Piggy Stayed Home," which features two pig halves floating in formaldehyde, and Marcus Harvey's "Myra," a 13-foot-high painting of a woman convicted in 1966 in the notorious British child killings known as the Moors murders. It was created from children's handprints, and in Britain, in created a furor and was splashed with ink and egg on the opening day of the show.