NEW YORK (Reuters) - New York City Mayor Bill de Blasio ended his two-year boycott of the annual St. Patrick’s Day parade on Thursday, joining in the world’s largest celebration of Irish heritage after organizers opened the event up to all openly LGTB marchers.

New York Mayor Bill de Blasio (C) marches along the Fifth Avenue during the St. Patrick's Day parade in New York March 17, 2016. De Blasio ended his two-year boycott of the annual St. Patrick's Day parade on Thursday, joining in the world's largest celebration of Irish heritage after organizers opened the event up to all openly LGTB marchers. REUTERS/Eduardo Munoz

De Blasio took part in the parade up Manhattan’s Fifth Avenue for the first time since his election after organizers agreed to allow a second lesbian, gay, bisexual and transgender group to march under its own banner.

For years organizers excluded LGBT groups from marching under banners identifying their sexual orientation, drawing the ire of critics who said the policy amounted to discrimination.

This year, the Lavender and Green Alliance, an Irish American LGBT organization, joined the procession along with marchers Out@NBCUniversal, a small LGBT group and affiliate of the parade’s television sponsor, which first participated in 2015.

The 255th edition of New York City’s parade was expected to draw more than 2 million spectators and 200,000 marchers, including more than 100 bands. This year’s event honors the 100th anniversary of the Easter Rising, the armed uprising during Easter Week of 1916 that led to Ireland’s independence.

George Mitchell, a former U.S. senator who helped negotiate the 1998 Northern Ireland peace accord, led the parade, agreeing to serve as grand marshal after being assured the gay rights dispute was resolved.

Months after de Blasio was elected in 2014, he became the first mayor in more than 20 years to decline to march in the parade, which traditionally draws many politicians eager to win over the city’s large Irish-Catholic population.

Others have also declined invitations due to the gay rights dispute, which led two sponsors, Heineken and Guinness, to drop out. They returned last year after the parade invited the NBC group.

“I’ve known so many members of the LGBT community who are Irish; who simply wanted to express their pride,” de Blasio said at a March 3 news conference. “And they wanted to know that they could do that like any other person.”

Boston’s parade had been involved in a similar dispute but last year opened its ranks to gay marchers.

“It’s a big change for us and change at any organization doesn’t come easy and takes a long time,” said John Lahey, a veteran parade organizer in his first year as board chairman, who is also president of Quinnipiac University in Connecticut.

Lahey said it was uncertain whether more LGBT groups would march in the future. He cited pressure to limit the size of the procession, which can last six hours. The Brehon Law Society, an Irish American lawyers group that previously boycotted in support of gay rights, is the parade’s only other new addition.