A sea of protesters poured through the frigid streets of midtown Manhattan today to protest a possible United States invasion of Iraq, as police hurriedly closed avenues to make way for the chanting, sign-waving, horn-tooting thousands.

The protesters arrived by car, train and bus from up and down the East Coast, with children and teenagers in tow, untold thousands offering a generational and ethnic rainbow.

"We've been so frustrated by this march to war," said Heather Hall, who drove from western Massachusetts and marched today with her son on Third Avenue.

Labor unions, too, took a big role. Five major national unions oppose the war.

"We are going to stop this war," said Dennis Rivera, leader of SEIU 1199, a powerful health care workers union that brought thousands of mostly black and Latino workers to the rally. "If they can march in Rome and Barcelona and London, we can march in New York, too."

New York was just a link in a global ring of protest as demonstrators from Canberra, Australia, to Los Angeles and Detroit took to the streets against a U.S.-led war with Iraq. About 1 million people protested in both Rome and London.

It appeared the New York protest drew at least 100,000 protesters. The police offered no official estimate. Thousands of protesters found their path blocked by a maze of police barricades and could not make their way to the demonstration site, eight blocks north of the United Nations on First Avenue.

Citing security concerns, Mayor Michael Bloomberg refused to grant the protesters a permit to march from the United Nations to a site eight blocks away, and city lawyers fended off court challenges to his decision last week. But the mayor's strategy had unintended consequences today, as tens of thousands of people organized de facto marches that forced police to close streets at a moment's notice.

Choke points dense with hundreds of protesters swelled on Second and Third avenues, and police officers on horseback galloped in to drive back the crowd, most of which appeared intent on peaceful protest.

The Harden-Mihich family -- husband, wife and 9- and 13-year-old daughters -- huddled by a metal police barricade at 53rd Street and Third Avenue, four blocks from the demonstration. They had traveled from Boston, but this was as close as they would get to the demonstration after hours of walking. Police had barricaded the street in every direction, penning in thousands of people.

"It's impossible to move," Sylvia Mihich said. "There's literally no way out."

A police lieutenant nearby was sympathetic. He noted that the city was on a heightened alert against terrorism and that the department had deployed a new security "package" that included sharpshooters and bomb-sniffing dogs.

But police officers found it took half an hour to travel a dozen blocks through the crowd. "It's nuts," he said. "If the city gave them a set march route down an avenue, you wouldn't have these problems."

Still, save for a few arrests and pockets of young protesters intent on civil disobedience, most of the crowd was festive and well-mannered. Groups of professionals -- health care workers, lawyers, university professors and labor activists -- rallied and then marched together toward the demonstration.

Their bobbing signs ranged from the standard "No Blood for Oil" to the inventively insulting -- "Drunk Frat Boy Drives Country into Ditch" -- to printed applause for the European nations that have challenged the U.S. position at the United Nations this week: "New York loves 'Old Europe.' "

This last sign was a reference to Defense Secretary Donald H. Rumsfeld's dismissal of Germany and France as part of an "old Europe" out of step with the United States and other European nations. Another sign, held aloft by a laid-off flight attendant, addressed those who link the terrorism attacks of Sept. 11, 2001, with a war on Iraq: "Don't Hijack Our Grief for War."

On a stage, a series of antiwar veterans, including 83-year-old folk singer Pete Seeger, singer Harry Belafonte and actors Susan Sarandon and Danny Glover, spoke to the crowd. A loud roar was reserved for South Africa's Archbishop Desmond Tutu.

"Peace! Peace!" Tutu cried in his familiar, high-pitched voice. "Let America listen to the rest of the world -- and the rest of the world is saying: 'Give the inspectors time.' "

The protesters were so hemmed in by barricades, and the day was so raw, that the standard rhetoric elicited no more than a mild response from the crowd. Many demonstrators emphasized that they held no brief for Iraqi President Saddam Hussein, whom they described as a thug and a dictator. But they insisted the war on Iraq would only complicate the United States' war on terrorism.

Quite a few drew a distinction between the war in Afghanistan after the terrorist strikes on New York and the Pentagon, which seemed a necessary struggle to push out a government harboring al Qaeda, and a potential invasion of Iraq.

"Iraq is ruled by an oppressive but secular dictatorship," said Mary Harron of Brooklyn. "Saddam Hussein has done many horrible things, and it's a shame we didn't overthrow him back in 1991. But this war will only push more people into the hands of the Islamic fundamentalists."

Protesters spoke of getting their news over the Internet or from alternative radio stations and community organizations. A labor activist from Silver Spring, Gene Bruskin, shifted from foot to foot as a river of protesters passed. In contrast to the Vietnam era, when they took a hawkish stance, many union locals oppose an Iraq war, he noted.

"It's come from the grass roots," he said. "We've had Webcasts around the world; locals from Seattle and Los Angeles have passed resolutions."

Back on 53rd Street, Dick Reilly rallied his sons for a final walk to the demonstration. A contractor, he was one of seven brothers who served in the military, and he is no pacifist. But this war makes little sense to him.

"Where's the exit strategy?" he asked. "So we go into Iraq and bomb and shoot, and Osama [bin Laden] has more recruits. How does it end?"

Dannette Sharpley, left, and Yolanda Carrington comfort each other as they listen to a speaker at the state Capitol in Raleigh, N.C., as part of worldwide demonstrations against a possible Iraq war. A antiwar demonstrator protests on New York City's East Side near United Nations headquarters in New York City. A New York City mounted police officer beats back an antiwar demonstrator as police cleared protesters from Third Avenue on New York's East Side.