(AP) Immigration rights advocates more than 500,000 strong marched in downtown Los Angeles, demanding that Congress abandon attempts to make illegal immigration a felony and to build more walls along the border.

The massive demonstration, by far the biggest of several around the nation in recent days, came as President Bush prodded Republican congressional leaders to give some illegal immigrants a chance to work legally in the U.S. under certain conditions.

Wearing white shirts to symbolize peace, marchers chanted "Mexico!" "USA!" and "Si se puede," an old Mexican-American civil rights shout that means "Yes, we can." They waved the flags of the U.S., Mexico and other countries, and some wore them as capes.

Saturday's march was among the largest for any cause in recent U.S. history. Police came up with the crowd estimate using aerial photographs and other techniques, police Cmdr. Louis Gray Jr. said.

Other demonstrations drew 50,000 people in Denver and several thousand in Sacramento and Charlotte, N.C.

Many protesters said lawmakers were unfairly targeting immigrants who provide a major labor pool for America's economy.

"Enough is enough of the xenophobic movement," said Norman Martinez, 63, who immigrated from Honduras as a child and marched in Los Angeles. "They are picking on the weakest link in society, which has built this country."

The U.S. House of Representatives has passed legislation that would make it a felony to be in the U.S. illegally, impose new penalties on employers who hire illegal immigrants, require churches to check the legal status of people they help, and erect fences along one-third of the U.S.-Mexican border.

Elger Aloy, 26, of Riverside, a premed student, pushed a stroller with his 8-month-old son at Saturday's Los Angeles march and called the legislation "inhumane."

"Everybody deserves the right to a better life," he said.

The Senate is to begin debating the proposals on Tuesday.

President Bush on Saturday called 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," Bush said in his weekly radio address, discussing an issue that had driven a wedge into his own party.

Bush sides with business leaders who want legislation to let some of the estimated 12 million undocumented immigrants stay in the country and work for a set period of time. Others, including Senate Majority Leader Bill Frist, say national security concerns should drive immigration reform.

"They say we are criminals. We are not criminals," said Salvador Hernandez, 43, of Los Angeles, a resident alien who came to the United States illegally from El Salvador 14 years ago and worked as truck driver, painter and day laborer.

Francisco Flores, 27, a wood flooring installer from Santa Clarita who is a former illegal immigrant, said, "We want to work legally, so we can pay our taxes and support the country, our country."

In Denver, police said more than 50,000 people gathered downtown at Civic Center Park next to the Capitol to urge the state Senate to reject a resolution supporting a ballot issue that would deny many government services to illegal immigrants in Colorado.

Elsa Rodriguez, 30, a trained pilot who came to Colorado in 1999 from Mexico to look for work, said she just wants to be considered equal.

"We're like the ancestors who started this country, they came from other countries without documents, too," the Arvada resident. "They call us lazy and dirty, but we just want to come to work. If you see, we have families, too."

Between 5,000 and 7,000 people gathered Saturday in Charlotte, carrying signs with slogans such as "Am I Not a Human Being?" In Sacramento, more than 4,000 people protested immigration legislation at an annual march honoring the late farm labor leader Cesar Chavez.

About 200 people protested outside a town hall-style meeting held by Rep. James Sensenbrenner, R-Wis., a leading sponsor of the House bill. He defended the legislation, saying he's trying to stop people from exploiting illegal immigrants for cheap labor, drug trafficking and prostitution.

"Those who do that are 21st-century slave masters, just like the 19th-century slave masters that we fought a civil war to get rid of," Sensenbrenner said at the meeting. "Unless we do something about illegal immigration, we're consigning illegal immigrants to be a permanent underclass, and I don't think that's moral."

Since Thursday tens of thousands of people have joined in rallies in cities including Milwaukee, Los Angeles, Phoenix and Atlanta, and staged school walkouts, marches and work stoppages.

The demonstrations are expected to culminate April 10 in a "National Day of Action" organized by labor, immigration, civil rights and religious groups.

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Associated Press writers Bob Jablon and Kim Nguyen contributed to this report.