The parade set off down Fifth Avenue on a cold Monday morning.
For police and firefighters, the St. Patrick's Day parade is an important tradition. Years ago, those were the only kind of jobs open to poor, Irish immigrants.
Today, though, the parade has become a flashpoint in the fight over gay rights.
Gay rights activists are angry because parade organizers forbid them to march with a banner identifying them as gay Irish-Americans.
This year, the new mayor joined them by boycotting the parade.
"I think the mayor is wrong, I have an honest disagreement with the mayor on this. Yeah I do think he's wrong," said Rep. Peter King. "Mike Bloomberg was 100% pro-gay in all his policies and he marched in the parade."
Cardinal Timothy Dolan also made it clear that on this issue, he's not happy with the mayor.
"The mayor's decision? Oh I'll talk to him privately about that one. This isn't a day for brawling," said Dolan.
But most elected officials and city leaders joined the mayor in boycotting this year's parade. One exception: Police Commissioner William Bratton.
"It's very important to me, heritage, as commissioner of the NYPD, which celebrates its great Irish heritage. There'll be thousands of officers marching today so I thought it was important to me marching with them also," said Bratton.
Gay activists said they appreciate the mayor's decision. But they actually wanted him to go further, to ban all uniformed police and fire from marching as well.
"If there was a Ku Klux Klan parade would de Blaiso allow cops to march in uniform in that parade? Never. This is equally as offensive as a Klan parade," said Mark Milano of the organization Irish Queers.
Many parade-goers actually said they thought gays should be allowed to march with a small banner like other groups.
But they thought gays went too far in their protest this year.
"I am actually a big supporter of equal rights for homosexuals but I'm also a big supporter of our safety in America. And I think it's really disgusting and disrespectful that they would boo the NYPD," said parade-goer Nicolle Mazzucca.
De Blasio's decision underscores political tensions over gay rights issues in the U.S. Boston Mayor Martin Walsh opted out of his city's parade Sunday after talks broke down that would have allowed a gay veterans group to march.
But Ireland's Prime Minister Enda Kenny joined the procession Monday, saying the holiday is about Irishness, not sexuality.
Thousands of green-clad spectators came out to in droves to watch bagpipers and marchers, and organizers of a float intended to promote diversity threw Mardi Gras-type beads to onlookers. Similar scenes played out in downtown Philadelphia and Boston.
In Michigan, parades were Sunday held in Bay City and Detroit, and on Monday a St. Patrick's Day Parade was scheduled in Cleveland. Cities from Savannah, Ga., to Montreal also hosted festivities over the weekend, and throughout the world landmarks were bathed in green floodlights.
Ireland's head of government, Enda Kenny, became the first Irish prime minister to attend Boston's annual St. Patrick's Day breakfast Sunday.
Kenny has resisted pressure, in both Ireland and America, to support the gay rights lobby's demand to have equal rights to participate in parades on St. Patrick's Day.
"The St. Patrick's Day parade (in New York) is a parade about our Irishness and not about sexuality, and I would be happy to participate in it," he said in Dublin before leaving for a six-day trip to the U.S.
In Ireland, St. Patrick's Day provides the launch of the country's annual push for tourism, a big part of the rural economy.
"To Irish people by birth or descent, wherever they may be in the world, and to those who simply consider themselves to be friends of Ireland, I wish each and every one of you a happy, peaceful and authentically Irish St. Patrick's Day," Irish President Michael D. Higgins, the ceremonial head of state and guest of honor at Monday's parade in Dublin, said in a statement.
Parade organizers in New York have said gay groups are not prohibited from marching, but are not allowed to carry gay-friendly signs or identify themselves as LGBT.
Some LGBT groups were to protest the parade along the parade route on Fifth Avenue on Monday. Others had planned to dump Guinness beer from the shelves of the Stonewall Inn, the birthplace of the gay rights movement, in protest of the brewer's plan to sponsor the parade, but that demonstration was canceled late Sunday after Guinness said in a statement that it had dropped its sponsorship.
Other beer companies joined the boycotts earlier, with Sam Adams withdrawing its sponsorship of Boston's parade and Heineken following suit in New York.
New York's parade, a tradition that predates the city itself, draws more than 1 million spectators and about 200,000 participants every March 17. It has long been a mandatory stop on the city's political trail, and will include marching bands, traditional Irish dancers and thousands of uniformed city workers.
The Associated Press contributed to this report.