The demonstrators marched under mostly clear blue skies with Spanish-language music blaring, street vendors selling ice cream and parents clinging to mischievous toddlers and the banners of their homelands.
The rallies, whose mood was largely festive rather than angry, were the latest in recent weeks in response to a bill passed in the House that would speed up deportations, tighten border security and criminalize illegal immigrants. A proposal that would have given most illegal immigrants a chance to become citizens collapsed in the Senate last week.
But Monday's gathering of tens of thousands of demonstrators in New York; Atlanta; Houston; Madison, Wis., and other cities also suggested that the millions of immigrants who have quietly poured into this country over the past 16 years, most of them Hispanic, may be emerging as a potent political force.
Over and over again, construction workers, cooks, gardeners, sales associates and students who said they had never demonstrated before said they were rallying to send a message to the nation's lawmakers.
Ruben Arita, a 30-year-old illegal immigrant from Honduras who joined the demonstration in Washington, said he was marching for the first time because he wanted to push Congress to grant citizenship to people living here illegally and to recognize their struggles and their humanity.
"We want to be legal," said Mr. Arita, a construction worker who has lived here for five years. "We want to live without hiding, without fear. We have to speak so that our voices are listened to and we are taken into account."
Academics and political analysts say the demonstrations represent the largest effort by immigrants to influence public policy in recent memory. And the scope and size of the marches have astonished politicians on Capitol Hill as well as the churches and immigrant advocacy groups organizing the demonstrations, leading some immigrant advocates to hail what they describe as the beginnings of a new, largely Hispanic civil rights movement.
Some Republicans in Congress say the rallieshave also recalibrated the debate on immigration legislation, forcing lawmakers to consider the group's political muscle.
"Immigrants are coming together in a way that we have never seen before, and it's going to keep going," said Jaime Contreras, the president of the National Capital Immigration Coalition, a group of business, labor and immigrant advocacy groups that organized the demonstration in Washington and helped coordinate the other national protests.
"This is a movement," said Mr. Contreras, who came to the United States from El Salvador as an illegal immigrant and is now a citizen. "We're sending a strong message that we are people of dignity. All that we want is to have a shot at the American dream."
Senator Sam Brownback, Republican of Kansas, who favors granting citizenship to illegal immigrants, said Monday: "I think everybody sees the immigrant community as an emerging force. I think everybody is quite sensitive that they don't want to be on the wrong side, politically, of this group."
But political analysts say it is not clear whether the fervor on the streets will translate immediately into a force at the ballot box.
In the 2004 presidential election, 18 percent of Hispanics voted, compared with 51 percent of whites and 39 percent of blacks, according to a study conducted by the Pew Hispanic Center. That reflects, in part, the large numbers of illegal immigrants, permanent residents and children under 18 in the Hispanic community who are unable to vote. But turnout has traditionally been low even among Hispanics registered to vote.
President Bush has called on Congress to create a temporary work program that would legalize millions of immigrants.
The demonstrations, while cheered by advocates for immigrants, have meanwhile fueled a sharp response from critics who have expressed outrage at the images of immigrants, some of them illegal, demanding changes in American laws.
Talk of the marches has been burning up the airwaves on talk radio and cable news networks and has appeared in Internet blogs and conservative publications. Rich Lowry, the editor of National Review, described the protests with marchers carrying foreign flags as "ominous" in "their hint of a large, unassimilated population existing outside America's laws and exhibiting absolutely no sheepishness about it."
Brit Hume, the news anchor on Fox News, described the marchers, particularly those carrying Mexican flags, as "a repellent spectacle."
But Senator Lindsey Graham, Republican of South Carolina, warned that politicians who chose to alienate this group did so at their own peril.
"I understand clearly that the demographic changes are real in America and how we handle this issue in terms of fairness will be very important for the future of both parties," Mr. Graham said Monday. "Those who believe that they have no political vulnerability for the moment don't understand the future."
The organizers of the protests called Monday a National Day of Action for Immigrant Justice, and the focus was on pushing for legislation that would legalize the roughly 11 million illegal immigrants believed to be living in the United States. And in Atlanta, where the police estimated that 30,000 to 40,000 people participated in the rallies, some marchers invoked the tactics and slogans of the civil rights era. Fabian Rodriguez, a 38-year-old illegal immigrant from Mexico, said he was tired of living in fear of being deported.
"We are in the situation that Rosa Parks was in several years ago," said Mr. Rodriguez, who works in the landscaping business. "Enough is enough."
In Houston, where thousands of immigrants chanted "U.S.A.! U.S.A.!" as they rallied, Staff Sgt. Jose Soto of the Marines marched in his blue uniform. He said he had fought in Iraq and was in Houston to visit his parents, who came to this country as illegal immigrants.
"I've fought for freedom overseas," said Sergeant Soto, 30, who plans to return to Iraq in July. "Now I'm fighting for freedom here."
In Madison, the crowds of demonstrators stretched nearly a mile as protesters headed to the Capitol. Maria Camacho, a 51-year-old Mexican immigrant, attended the march with her husband and daughter. Wearing a white sweater with an American flag, she held up a sign that read, "No human being is illegal."
No rally was more diverse than New York's, where the thousands who converged at City Hall Park were greeted in Spanish, Chinese, French and Korean, and heard invocations by a rabbi and the leader of a Buddhist temple.
"We are inseparable, indivisible and impossible to take out of America," Chung-Wha Hong, executive director of the New York Immigration Coalition, told a spirited crowd that included hotel housekeepers from El Salvador, Senegalese street vendors, Chinese restaurant workers and Mexican laborers.
In Washington, demonstrators carried children on their shoulders, ate popcorn and draped themselves in the banners of their homelands as they cheered Senator Edward M. Kennedy, Democrat of Massachusetts, who told them that the Rev. Dr. Martin Luther King Jr. had spoken here in 1963, and a host of other speakers, including John J. Sweeney, president of the A.F.L.-C.I.O., and Cardinal Theodore E. McCarrick of Washington.
Across the street from the rally, about half a dozen people held signs that read, "Illegals Go Home."
But the small counterprotest failed to douse the spirits of the demonstrators, many of whom seemed almost giddy with their newfound sense of political power.
"Today we march," they chanted. "Tomorrow we vote!"