Hundreds of thousands of demonstrators are expected in New York to protest over US President George W. Bush's policies the day before the Republican convention opens in the city.
The march will pass the Madison Square Garden convention site in a circular route through Manhattan but activists have vowed to defy a ban on rallying in Central Park, the city's largest open space, later in the day.
Two hours before the noon march on Sunday organised by the United for Peace and Justice coalition, thousands filled the sidewalks and streets as the heat and humidity pushed the temperature to almost 29 degrees 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 US-led invasion and occupation of Iraq last year.
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 Centre site destroyed in the September 11, 2001 attacks by Islamic militants.
Today's march takes place as the 50,000 delegates and other visitors converge on New York for the four-day gathering 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.
Security around the arena has been called the tightest in the history of U.S. political events with thousands of police officers and Secret Service agents on guard.
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.
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.