The rally brought the return to major street action byimmigration activists, who turned out hundreds of thousands of protesters in marches and rallies in 2006. After an immigration overhaul measure was defeated in Congress in 2007, the pace of enforcement raids picked up and many immigrants, especially those without legal status, preferred to lay low.
But immigrant advocates decided to gamble by calling the march, to give a show of force that might impress Mr. Obama and also to vent the frustration of many immigrants who have taken to heart his repeated promises that he would move an immigration bill in Congress by early this year.
Mr. Obama addressed the crowd via a videotaped message displayed on huge screens, promising to keep working on the issue but avoiding a specific time frame.
“I have always pledged to be your partner as we work to fix our broken immigration system, and that’s a commitment that I reaffirm today,” Mr. Obama said.
He expressed his support for the outline of an immigration bill presented last week by Senator Lindsey Graham, Republican of South Carolina, and Senator Charles E. Schumer, Democrat of New York. While pledging to help build bipartisan support, Mr. Obama warned, “You know as well as I do that this won’t be easy, and it won’t happen overnight.”
But speaker after speaker rose to demand immigration legislation sooner rather than later, leaving aside any mention of the acrid political environment in Washington in the aftermath of the health care battle.
“Every day without reform is a day when 12 million hard-working immigrants must live in the shadow of fear,” said Representative Nydia M. Velázquez, a Democrat from New York who is the chairwoman of the Congressional Hispanic Caucus.
“Don’t forget that in the last presidential election 10 million Hispanics came out to vote,” she said. She told the crowd to tell lawmakers “that you will not forget which side of this debate they stood on.”
Representative Luis V. Gutierrez of Illinois, a Democrat who has been a leader of the immigrants’ movement, said he was optimistic that Mr. Obama would try to get an immigration bill this year.
“I see a new focus on the part of this president,” Mr. Gutierrez said. “That’s why we are here to say we are not invisible.”
The urgency was echoed by church leaders who spoke, including Roman Catholic Cardinal Roger M. Mahony of Los Angeles, and Reverend Samuel Rodriguez, the leader of the National Hispanic Christian Leadership Conference, the largest organization of Latino evangelical churches.
“The angst and trepidation in our communities is unprecedented,” Mr. Rodriguez said. He compared the mood among Latinos to the hard days of the civil rights movement. “This is our Selma,” he said.
Echoing that thought were an array of African-American leaders who turned out for the event. Speakers included the Rev. Jesse Jackson; Benjamin T. Jealous, president of the N.A.A.C.P; Cornel West, a Princeton scholar, and Marc H. Morial, a former mayor of New Orleans and the president of the National Urban League.
Ali Noorani, executive director of the National Immigration Forum and a leading organizer of the event, said that rallies were planned in several cities on April 10, the last day of the Congressional recess. On May 1, Mr. Noorani said, immigrant groups would release a report card of every lawmaker and where they stand on the immigration overhaul.
Senator Robert Menendez, Democrat of New Jersey, said he thought an immigration bill could pass at the end of the year, after the storm of the November elections had passed.
The crowd, overwhelmingly Latino immigrants, arrived on buses from California, Ohio, Texas, Michigan, Colorado and many other places. Unions brought thousands of members, including dozens of workers from a meat-packing plant in Tar Heel, N.C.
While a few demonstrators waved flags from other countries, most flew American flags overhead, recalling the negative reaction from American voters to earlier protests where Mexican flags dominated. Farm workers from Florida held one billowing flag overhead and propped it with sticks, forming a tent.
In the crowd, frustration with Mr. Obama was strong. Rudy Romero, 19, and Andrea Rentaria, 23, said they boarded buses early Friday in Colorado with 54 other people, and 36 hours later, arrived in Washington. They said they were disappointed with the pace of progress on immigration.
“We’ve been waiting for so long,” Mr. Romero said. “I know it takes time, but a promise is a promise. We are demanding it today.”
Ms. Rentaria added, “We want to step up and say, ‘Hey, wake up. We’re here. We’re still waiting. We’ve given you time to settle in. When is this going happen?’ ”
“I understand you have to take care of health care,” Ms. Rentaria said. “As soon as we’re done with that,” she said, immigration should be next.
Although there were a few jeers for Mr. Obama during a morning rally, the crowd roared when he appeared on video.
Adrian Vasquez, 32, held up a sign reading “Support Our President, Immigration Reform Now!” Mr. Vasquez, who has been in the United States for 20 years and is now an illegal immigrant, admitted that the push for an overhaul “could not come at a worse time” for Mr. Obama.
But he said, “I’m eager for change. I think we can get it done.”