In a decisive break from his two predecessors, New York Mayor Bill de Blasio confirmed Tuesday that he will not march in the city’s 2014 St. Patrick’s Day Parade due to its policy of effectively excluding openly gay and lesbian groups. Speaking to reporters at City Hall, the newly minted mayor said he plans to maintain the same stance he held as public advocate. “I simply disagree with the organizers of that parade in their exclusion of some individuals in this city,” he told reporters.

The subject came up in light of an open letter to de Blasio penned by numerous LGBT advocates, including de Blasio’s successor as public advocate, Letitia James. Published on Monday in Gay City News and on the website Irish Queers, the letter calls on the mayor to prohibit uniformed city personnel from marching in the parade. The letter’s signatories cite a 1993 court decision declaring the event a “religious procession,” one whose policies are constitutionally protected. The activists argue that participation by uniformed city police and firefighters violates the city’s human rights law:

“The presence of uniformed police and firefighters in such a procession sends a clear signal to LGBTQ New Yorkers that these personnel, who are charged with serving and protecting all New Yorkers, do not respect the lives or safety of LGBT people. It confirms the practice of the NYPD and FDNY at times of targeting certain communities for discrimination. What’s more, it betrays the current work of high level government agencies and human rights advocates working internationally against the current wave of extreme anti-LGBTQ legislation and discriminatory practices occurring in countries such as Nigeria, Uganda and Russia.”

As of Tuesday afternoon, the letter has been signed online by more than 130 groups, activists, academics, artists, city officials and civil rights attorneys.

Despite the letter, de Blasio made it clear at the press conference that he believes uniformed public servants have a right to march if they choose to do so. At the same time, his own decision not to attend is a departure from Mayors Michael Bloomberg and Rudolph Giuliani, both of whom happily participated in the storied Fifth Avenue procession, which dates back to 1762. De Blasio will be the second New York City mayor in recent history to boycott the parade, following David Dinkins, who did so in the early 1990s.

New York City Saint Patrick Day Parade Inc., which organizes the event, did not immediately respond to a request for comment. Organizers, however, have long insisted that the parade is not anti-gay. While individual gays and lesbians may march, they say, participants are prohibited from displaying pro-LGBT signage and attire. Organizers say such displays detract from the event’s message of celebrating Irish heritage.

In a statement Tuesday, Bill Donohue, president of the Catholic League, said de Blasio’s absence will be no great loss to the parade. “Personally, I am delighted,” he said. “I lead the Catholic League contingent every year, and I do not want to march with a public official who does not want to be associated with Irish Catholics.” Donohue went on to call descriptions of the parade as anti-gay “a great myth,” pointing out that the event also prohibits anti-abortion signage. “The parade is not about homosexuals, or abortion, or anything other than honoring St. Patrick,” he said.

The gay rights activists, meanwhile, are continuing to collect signatures, hoping to put pressure on de Blasio to prohibit uniformed city officials from participating in the event. Read the full letter here.

The 2014 St. Patrick’s Day Parade takes place on Monday, March 17, beginning at 11 a.m.

Got a news tip? Email me. Follow me on Twitter @christopherzara.