They marched against US policy in the Middle East, against the war in Iraq, against the Bush administration's environmental policy and the Republican Party's opposition to abortion and gay marriage.
The marchers were close to the site of the 11 September attacks
They marched against what they saw as a transparent ploy by President George W Bush and the Republican Party to exploit the tragedy of 11 September for political gain.
But one thing united the tens of thousands of protesters who marched for block after block through midtown Manhattan: Their desire to vote George W Bush out of office.
March organisers had predicted that 250,000 would take part in the protest.
They said that the turn out exceeded their wildest expectations.
The march moved very little in the hours after organisers started on the route towards Union Square as tens of thousands of marchers flooded in from side streets.
The marchers included children, young couples, students, the middle aged and the elderly. One woman sat in her wheelchair in Union Square holding a sign saying, "I'm 98 and I'm outraged!"
Barbara, who describes herself as a middle-aged schoolteacher, had never been to a protest march until Sunday.
She held a sign saying that she was a former Republican against George Bush.
Many in the march felt they had been misled in the 'war on terror'
She defected from the party in after the first term in office of the current president's father, George HW Bush, when religious and social conservatives Pat Buchanan and Pat Robertson spoke at the party's 1992 convention.
"They sounded like dangerous guys, but this George Bush has them in the closet. And they run the party," she said.
"[President George W Bush] has given us record deficits. When Osama Bin Laden is in Afghanistan, he went to war in Iraq. It's a mess," she said.
"We have to get rid of George Bush. We can't tolerate another four years of his international or domestic policies," she said.
Four more years or four more months
Just blocks from the Madison Square Garden, a group of protestors decided to stay at home in their apartment overlooking the route.
They hung "George Bush is the anti-Christ" signs in their window, and a cheer went up from the marchers below as they played the John Lennon song, Give Peace a Chance, from speakers propped up in their windows.
The protest was peaceful. There was a strong police presence, but most were not in riot gear. There were few confrontations.
Protesters wanted the Republicans to know they were not welcome
On the eve of Sunday's massive march, New York City Police said more than 300 protestors had been arrested and more than 50 more were arrested during the first hour of the march.
Across the street from Madison Square Garden, Bush supporters held signs mocking the protestors with slogans such as "Communists for Kerry" and chanted, "Four more years!"
The protestors shouted back, "Four more months!"
A sign at Madison Square Garden welcomed the Republicans, but many protestors wanted the party to know they were not welcome in New York.
People of all ages and different backgrounds took part
Ray Lyman worked in a building across the street from the south tower of the World Trade Centre. The building was heavily damaged and is still covered in a black shroud.
He said the Republican Party's decision to hold its convention in New York is insulting and emotionally offensive. "It feels like a complete violation," he said.
"I don't believe in the way that Bush and the Republican Party are using the tragedy of 9/11 as their campaign platform," he said, adding, "They are using 9/11 as an excuse to further their own right-wing agenda."
Qualified support for Kerry
Four years ago, the protests at the Democratic and Republican conventions were made up largely of Americans who felt alienated from traditional party politics and saw little difference between the two mainstream parties.
But at this march, the political theme could be summed by "Anybody but Bush", even if that means electing John Kerry who voted for the war in Iraq, which they overwhelmingly oppose.
Stephanie Jennings came all the way from San Diego, California, to march in New York.
She carried a sign calling for voters in swing states to vote for the "lesser of two warmongers".
Four years ago, she voted for Ralph Nader. She still believes both parties represent corporate interests over the interests of ordinary people.
The marchers were unified by their opposition to President Bush
"We're here because of Bush policies and the war," she said, but she is also not a strong supporter of the Democratic Party and its presidential candidate, John Kerry.
If Mr Kerry has a large lead in California, she plans to vote for the Green Party candidate.
"I need the Democrats to know they can't take my vote for granted," she said.
But she wants John Kerry to win. Under Bush policies, "we are on the precipice. It's a really dangerous time," she said. "Four more years of this, and I fear for the world."