Hundred of thousands of people converged in cities around the nation for pro-immigrant rallies as the U.S. Senate prepares to debate legislation increasing penalties for undocumented aliens and the people who hire them.

Police tell CBS News.com that about 500,000 people were rallying the streets of downtown Los Angeles on Saturday at 11:30 a.m. – the height of the demonstration. By early afternoon, the crowds began to thin out. At 2 p.m., 200,000 people remained, according to the police.

The Los Angeles Police Department says the march was peaceful, and there were no reports of arrests.

The protestors massed on the steps of City Hall, perched in trees and atop bus kiosks. Many of the marchers wore white shirts and waved U.S. flags. Some also carried the flags of Mexico and other countries, or wore them as capes.

The rally in Los Angeles is one of many large pro-immigration rallies taking place Saturday in cities across the nation. In Denver, an estimated 50,000 people crowded Civic Center Park, near the state capitol.

Denver police spokesman Sonny Jackson says only a few thousand people were expected at the protest, but officers reported no problems and the four-hour rally ended without incident.

Speakers during the rally ridiculed the Republican party, telling participants that "they're not on our side and they're pitting Americans against us."

The House of Representatives passed legislation in December that would make it a felony to be in the U.S. illegally, impose new penalties on employers who hire illegal immigrants and erect fences along one-third of the U.S.-Mexican border. The Senate is to begin debating the proposals on Tuesday.

President Bush, bracing for protests and a Senate showdown, called Saturday for legislation that does not force America to choose between being a welcoming society and a lawful one.

"America is a nation of immigrants, and we're also a nation of laws," Mr. Bush said in his weekly radio address about the emotional immigration issue that has driven a wedge in his party.

"As we debate the immigration issue, we must remember there are hardworking individuals, doing jobs that Americans will not do, who are contributing to the economic vitality of our country," Mr. Bush said.

The president's message was echoed in Denver, where one Colorado farmer told CBS affiliate KCNC-TV, "Let them come here legal, quit chasing them."

"Let them be comfortable here while they're working and doing our work that nobody else will do," he said of immigrant workers.

Smaller demonstrations took place across the nation on Friday. The rallies are expected to culminate in a "National Day of Action" April 10 organized by labor, immigration, civil rights and religious groups.

In Los Angeles, more than 2,700 students from at least eight Los Angeles high schools and middle schools poured out of classrooms in protest and visited other campuses, imploring students to join them.

In Phoenix, police said 20,000 demonstrators marched on Friday to the office of Republican Sen. Jon Kyl, co-sponsor of a bill that would give illegal immigrants up to five years to leave the country. The turnout clogged major thoroughfares in what officials said was one of the largest protests in the city's history.

People also protested outside Kyl's Tucson office.

Kyl pointed out that most were speaking out against the House bill making it a felony to be an illegal immigrant, not his bill, which would also step up border enforcement and create a temporary guest-worker program.