Hundreds of thousands of people have rallied in New York against President Bush, as his Republican Party gathers for its national convention.
The marchers occupied a stretch of road more than a mile long
Speakers on the first day of the convention will include the former mayor of New York, Rudolph Giuliani, and Senator John McCain.
A massive security sweep has virtually sealed off the convention venue.
Police officers with dogs, bomb-detecting equipment and riot gear have closed off the surrounding streets.
Delegates to the convention, due to take place in Madison Square Garden, are being greeted with a list of prohibited items that includes guns, explosives, fireworks and knives.
Sunday's march, which went past the Madison Square Garden convention site, was led by activists including filmmaker Michael Moore and former presidential hopeful Jesse Jackson.
Police gave no official crowd estimate. One law
enforcement official put the crowd at 125,000 people but organisers said it was more
than 500,000, the Associated Press reported.
There were no immediate reports of incidents or injuries during the march, which followed smaller demonstrations over the weekend that led to nearly 300 arrests.
Four police helicopters buzzed overhead during the march.
After weeks of legal wrangling, protesters have been denied the right to march across Central Park, but some had vowed to defy the ban, raising the prospect of a confrontation with police.
The Republican convention is being held just a short distance from the site of the World Trade Center, which was destroyed in the 11 September 2001 attacks - and just a few days before the third anniversary of those attacks.
The date marked the beginning of what the Bush administration calls its war on terror, and the president has made this a centrepiece of his re-election campaign.
But our correspondent says Mr Bush's opponents believe that, by holding the convention in New York, he is exploiting the tragedy for his own political ends.
CONVENTION: DAY ONE
New York City Mayor Michael Bloomberg
Roll calls start on nominations for president and vice president
Senator John McCain of Arizona
Former New York City Mayor Rudolph Giuliani
The president is not due to arrive at the convention until Wednesday.
He will spend one night in New York and accept the Republican nomination as presidential candidate, before heading for the election battlegrounds of Pennsylvania, Ohio and beyond.
The opening day of the convention is reportedly intended to focus on Mr Bush's leadership
in the "war on terror", with a tribute to families of the 11 September 2001 attacks.
In addition to Mr McCain and Mr Giuliani, speakers include New York City Mayor Michael Bloomberg.