Demonstrators carrying colourful banners and signs marched up one of New York's main avenues on Sunday to protest President George W. Bush's policies over the Iraq war and the economy the day before the Republican convention opens.
The marchers, estimated by the United for Peace and Justice coalition to reach more than 200,000, were to pass the Madison Square Garden convention site on Eighth Avenue as Republicans and visitors converge on New York for the gathering that will end with Bush's re-nomination for president.
The start of the march took on a carnival atmosphere with people carrying large banners, shouting "No More Bush" and beating drums. The heat and humidity pushed the temperature to almost 85 degrees Fahrenheit (29 Celsius).
Many held banners and signs such as "Say No to the War Economy," "Bush Must Go" and "Bush lied, thousands died" in opposition to the war. The Bush administration said it invaded Iraq to rid Saddam Hussein of weapons of mass destruction that threatened America's security but no stockpiles were found.
The circular route takes the marchers through central Manhattan and activists have vowed to defy a ban on rallying in Central Park, the city's largest open space, later in the day.
Protesters were denied a permit to gather in Central Park after the march on the grounds city officials feared damage to the grass. But organisers of some groups have urged people to make their way to the park for a "people's picnic" after dispersing peacefully at the end of the march.
Organisers and a series of prominent speakers including civil rights activist Jesse Jackson, film maker Michael Moore and some New York elected officials boosted the protesters with speeches before the start of the event.
"Today, we send our message," long-time activist Leslie Cagan said. "We come from all walks of life ... from cities and towns across this nation and together we will march and in a resounding, clear voice we will say no to the Bush agenda."
Cagan and other speakers called for the United States to immediately withdraw troops from Iraq.
Kelly Doherty, a military police sergeant who served in Iraq for a year and helped establish "Iraq Veterans Against the War," said Iraqis and Americans had been "dehumanised" by the conflict.
"This is also dehumanising United States troops who are also having their sense of patriotism and loyalty perverted and used by an administration that would send our women and men to fight, die and kill for lies," Doherty said.
Security around the arena has been called the tightest in the history of US political events with thousands of police officers and Secret Service agents on guard.
Since Thursday, police have arrested more than 300 people in an array of demonstrations in which activists have stood naked in front of Madison Square Garden, swarmed central Manhattan in bicycles and solemnly rang bells at the World Trade Center site destroyed in the September 11, 2001 attacks by Islamic militants.
Streets were closed and concrete barriers put in place to deter car or truck bombs amid warnings from the government that al Qaeda or other groups might attack the United States during the election season. Sunday's march takes place as the 50,000 visitors come to New York for the four-day convention to nominate the president for a second term in the White House. He will face Democratic candidate Senator John Kerry of Massachusetts in the November election.