Irish pride on display at the New York City St. Patrick's Parade

MIDTOWN, Manhattan (WABC) -- New York City hosted its annual St. Patrick's Day Parade on Saturday, bringing together tens of thousands of marchers in one of the largest American celebrations of Irish heritage.

The six-hour procession of green-clad marchers up Fifth Avenue, past St. Patrick's Cathedral, is one of the city's oldest events, with its earliest iterations dating back more than 250 years.

It was held on Saturday this year because St. Patrick's Day falls on a Sunday. About 150,000 marchers took part in the parade, which began at 44th Street and continued to 79th Street.

The theme this year is supporting immigrants and immigration. The grand marshal is Brian O'Dwyer, an immigration attorney and activist.

"No community has done more to make New York City great, no community for so long has helped build us into the greatest city in the world than the Irish community," said Mayor Bill de Blasio, speaking at a breakfast before the parade.

Through its history, the parade has also often had a political element. In the 1970s and 1980s, as sectarian violence flared in Northern Ireland, there were controversies over the inclusion of groups supporting the militant wing of the Irish Republican Army. A banner reading "England get out of Ireland" has flown in the parade since the 1940s.

This year's march took place amid a new set of questions about relations between the United Kingdom and Ireland.

"When the Irish take to the streets this Saturday for the 258th St. Patrick's Day Parade, our thoughts will take us far beyond the festivities on Fifth Ave. to Washington, D.C., and to the British Parliament in London," O'Dwyer wrote in an editorial in the Daily News this week.

British lawmakers are struggling to find a way to exit the European Union without disrupting the two-decade old peace accords that created an open border between the Republic of Ireland, which is in the E.U., and Northern Ireland, which is in the U.K.

This week, with a March 29 deadline looming, British lawmakers voted to seek to delay Brexit for at least three months. But the possibility exists that the line between the two parts of Ireland, which has been unguarded for 20 years, will once again become hardened with vehicle checkpoints, with trade rules and tariffs in force.

O'Dwyer said Irish-Americans are ready to mobilize politically to oppose any arrangement that leads to a restoration of the hard border that once split the emerald isle.

"It was Irish-American activists who pressured former President Bill Clinton, over the objections of London and the U.S. State Department, to grant a visa to Sinn Fein's Gerry Adams that set in motion the process that led to the Good Friday Agreement, which ended the sectarian violence known as the Troubles and opened the border between the two Irelands for the first time since partition at the time of Irish Independence in 1921," he wrote. "We have been talking to leaders on both sides of the Atlantic to let them know that the same Irish-American activists who pressured the Clinton administration are ready to saddle up again and fight against a post-Brexit trade deal between the U.S. and Great Britain if a hard border is restored."

For most at the parade, the political debate over the future Northern Ireland was expected to take a back seat to the pageantry of the parade.

Popular marching groups include the pipes and drums corps for the Emerald Societies at the New York police and fire departments and the 1st Battalion, 69th Infantry Regiment, of the New York Army National Guard, which has led off the parade since 1851.

The parade followed Mass at St. Patrick's Cathedral, where Cardinal Timothy Dolan greeted marchers.

(The Associated Press contributed to this report.)

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