Hundreds of thousands of people crowded the streets of downtown Los Angeles on Monday in support of immigration reform, joining more than a million people who took a day off from work and class to demonstrate nationwide.

Protesters packed the streets of downtown displaying American colors and U.S. flags, and holding signs.

The demonstrations were colored with renditions of the national anthem, chants in both English and Spanish, and dances.

An estimated 250,000 gathered in front of City Hall earlier in the day. That number grew to 400,000 in the afternoon as demonstrators marched down Wilshire Boulevard from MacArthur Park toward La Brea Avenue.

As part of the protest, undocumented workers and their supporters both skipped work and participated in a boycott in an attempt to demonstrate the importance of immigrants to the economy.

The demonstrators had vocal support from figures like Los Angeles Mayor Antonio Villaraigosa, who was present at the rally.

“We come to work; we come for a better life; we come to participate for the American dream,” he told the protesters, according to the Los Angeles Times.

The crowds on city streets Monday contrasted with emptier businesses, stores and restaurants throughout country.

In downtown Los Angeles, about one-third of small businesses were closed because of the rally, and some restaurants on campus reported closing early and being slightly under-staffed.

There were also rallies across the country, with a large contingent of 400,000 demonstrators in Chicago and tens of thousands of others in New York, San Jose, Calif., and Houston.

The large mobilization was sparked by H.R. 4437, legislation passed in the House of Representatives in December that would criminalize undocumented immigrants, as well as those who aid them, and increase enforcement along the U.S.-Mexico border.

Though this specific legislation has stalled in Congress, policy-makers are in the middle of discussions on the largest immigration reform in two decades.

In the Los Angeles Unified School District, about 75,000 students were absent, which is about one quarter of the total student body.

Though Westwood and UCLA were not impacted to the degree that downtown was, signs of the rally’s effects were present.

Both Rubio’s and Panda Express in Ackerman Union closed early because of staffing shortages.

But the campus itself was not significantly affected, said Phil Hampton, a UCLA spokesman.

“The ‘Day Without Immigrant’ activities has had a minimal effect on campus services,” he said. “There have been some absences reported, but we don’t know how related they are to ‘Day Without Immigrant’ activities.”

Where there were absences, UCLA was able to function normally without a problem, he said.

Employees had been warned that if they called in sick Monday they might be required to show a doctor’s note, said UCLA Health Sciences spokeswoman Roxanne Moster.

Business representatives in Westwood Village also said they were not significantly affected. Though clientele may have dipped slightly, and there were scattered shortages of labor, a sampling of 15 to 20 Westwood business managers said they did not report significant losses.

Last week, California lawmakers agreed to cancel House and Senate sessions for the day, a decision Republican lawmakers protested Monday.

Senate Minority Leader Dick Ackerman, R-Tustin, said the resolution passed last week endorsing Monday’s protest “blurred the distinction between legal and illegal immigration.”

The White House expressed desire to see change in the form of legislation.

“The president is not a fan of boycotts,” said press secretary Scott McClellan.

“People have the right to peacefully express their views, but the president wants to see comprehensive reform pass the Congress so that he can sign it into law.”

There are an estimated 11 million undocumented workers in the U.S., and about 14 percent of the American population is Latino.

The growing percentage of Latino voters in this country means that such vocal opposition could have long-term political consequences.

“(The protest) doesn’t seem to me likely to have much effect on the California primaries that are coming up in June,” said UCLA political science Professor David Sears. “As far as the congressional elections in the fall, it’s likely to mobilize the Latino vote to some extent.”

The bill triggered much of the current mobilization within immigrant communities. But much of its effects depends on how the protests are perceived, Sears said.

Protesters carried Mexican flags in the large-scale rallies in March, causing some to question the loyalty of the immigrants to the United States.

This time, “there was a lot of emphasis on carrying American flags” during the preparation of the protest, Sears said.

With reports from Bruin wire services.