An additional 20,000 or so demonstrators took to the streets of the city Monday, using drums and horns and chants to raise a ruckus about the Republicans who have come to town for the week.

In the first march, at midday, more than 15,000 tenants, homeless families, migrant workers, and organizers for immigrants and poor people snaked one mile from Union Square -- that century-old wellspring of protest -- to Madison Square Garden, site of the Republican National Convention. Met at the Garden by the pulsating Puerto Rican music known as bomba, the marchers presented themselves as the face of those left behind -- the rising number of Americans, many of them from communities of color, who fell into poverty over the past three years.

"It's about concrete issues today, poor people have suffered," said Monami Maulik, 29, a member of the Still We Rise/Racial Justice 911 coalition, which organized the march. "There's a demand for comprehensive commitment for education, jobs and health care."

As was true Sunday, when more than 200,000 people flowed through Midtown to protest President Bush's policies, the marchers Monday were cheered by New Yorkers leaning out windows with noisemakers and signs. Jonathan Rivas, 35, from Queens, watched the demonstrators while dressed in his white kitchen uniform.

"They represent us and what we can't say," Rivas said. "We have to work. We can't take the day off."

Just before evening, a march led by the Kensington Welfare Rights Union moved from a plaza outside the United Nations to Madison Square Garden. The protesters had no permit, but police let them pass.

"There's a war against the poor, a war against our right to health care . . . all under the name of national security," said Ajamu Baraka, a human rights activist in Atlanta.

For several days now, protests have seemed to spring up in every corner of Manhattan, from political theater in Central Park to protesters rappelling down the face of the Plaza Hotel. Relatively little violence has accompanied the demonstrations, but police have made 531 arrests, the vast majority on misdemeanor charges, according to the Manhattan district attorney.

The largest number of arrests, 260, came Friday evening, when police moved in on Critical Mass, a group of bikers who cycle around Manhattan and temporarily block intersections. The riders have taken these unofficial trips around the city for years.

On Sunday, police arrested a different group of protesters who were loudly protesting Republicans on the streets of the theater district. The police detained some bystanders and legal observers in that action, reporters on the scene said.

"Our sense is that most of the arrests are not justified," said Chris Dunn, associate legal director for the New York Civil Liberties Union.

Attorneys with the National Lawyers Guild say that a number of these demonstrators, even those nabbed on misdemeanor charges, spent more than 20 hours in police wagons, precincts and holding cells before being released with what are known as "desk appearance tickets."

Paul Brown, a senior police official, insisted that no protester had waited more than 12 hours before being released. Officers had been "restrained and professional throughout," he said.

A policeman casts a wary eye as members of the coalition Still We Rise protest on behalf of low-income New Yorkers, who they believe are ignored by leaders.