The noon rally in Manhattan's Union Square is one of dozens around the nation on Sunday marking May Day. Marchers plan to walk down Broadway to lower Manhattan for a second rally there.
May 1 is a traditional date for pro-labor demonstrations.
Immigration advocates latched onto the tradition in 2006.
Labor advocates are protesting efforts to curtail the right to collective bargaining in Wisconsin, Ohio and other states.
May Day rallies also are taking place around the world on Sunday.
About 200,000 workers gathered in Istanbul's Taksim Square in Turkey.
Across Germany, some 420,000 people took to the streets to demand fair wages and better working conditions.