Flanked by police in riot gear, the protesters moved through the fortified city, loudly and exuberantly chanting slogans such as ''No more years.'' They accused the White House of waging an unjust war in Iraq, making the country poorer and undermining abortion rights.
There were no reports of major violence and about 200 scattered arrests, most of them unconnected to the main protest.
Police gave no official crowd estimate, though one law enforcement official, speaking on condition of anonymity, put the crowd at 120,000; organizers claimed it was roughly 400,000. In either case, experts said it was the largest protest ever at a U.S. political convention.
The five-hour march snaked in a circular route around midtown Manhattan, shutting down dozens of blocks and bringing out hordes of police.
''They chose New York, where they're universally hated,'' said writer Laurie Russo, 41, from New York. ''They should have gone somewhere they're more welcome. They exploited 9-11 by having it in New York at this time.''
In the largest set of arrests, some 50 protesters on bicycles who stopped near the parade route were carted away in an off-duty city bus. Fifteen people were arrested and two police officers were injured when someone set a paper dragon float afire near Madison Square Garden, and nine demonstrators were charged with assault after trying to stop police from arresting the culprit, authorities said.
In a smaller protest, police used clubs briefly to disperse a handful of demonstrators holding a ''kiss-in'' not far from Times Square. There were no immediate details about injuries or arrests.
''There's been a few minor arrests,'' Mayor Michael Bloomberg said. ''It has been peaceful.''
Residents leaned from windows along the demonstration route to shout their support. Scattered opposition was visible only around Madison Square Garden, where the GOP convention opens Monday. Some early convention arrivals looked across police lines, shouting at demonstrators: ''Go home!''
''I hope this shows the world that they're not alone in their hatred of George Bush,'' said Alan Zelenki of Eugene, Ore., who planned for three months to attend this week's protests.
The causes varied as much as the people shouting support: immigrants' rights, gay rights, universal health care, the Palestinian cause, an end to the killing in Sudan. Tracy Blevins, a biomedical researcher who recently left New York for Houston, dyed her Maltese pink and carried the little dog in a baby pouch to advocate peace.
Some demonstrators batted around a 6-foot-wide inflatable globe. One sign echoed Democratic nominee John Kerry's Vietnam-era remark: ''How do you ask a soldier to be the last person to die for a lie?''
The protest organizers, United for Peace and Justice, had sued unsuccessfully to force the city to allow a rally in Central Park. City officials said such a rally would damage lawns.
Earlier, ''Fahrenheit 9-11'' director Michael Moore told demonstrators that ''the majority of this country opposes the war.''
The majority never voted for the Bush administration,'' he said, ''and the majority are here to say, 'It's time to have our country back in our hands.'''
About 300 protesters earlier were arrested, and experts said the size of Sunday's demonstration was unmatched.
''I can't remember anything this big in history,'' said Sidney Tarrow, a professor of government and sociology at Cornell University. ''In 1968 (at the Democratic convention in Chicago) it was much more violent and there were many fewer people.''
Security was tight on the city's subways and trains, with police making frequent walks through cars and eyeing passengers up and down. The New York Daily News made a pitch for calm, publishing a front-page editorial Sunday.
''Play Nice,'' said the headline.