The marchers chanted "Si se puede," a Mexican-American rallying cry that means "yes, we can."

They hope to block a bill already passed by the House of Representatives that would make it a felony to be an illegal immigrant in the United States. The bill, which is not yet law, would also penalize those who help or hire illegal immigrants, and erect fences along one-third of the U.S.-Mexico border.

A spokesman for the Los Angeles Police Department says about 500,000 people marched Saturday.

Los Angeles Mayor Antonio Villaraigosa is the son of an immigrant, and he spoke in English and Spanish at the downtown rally. He later told reporters that people who come to his city to work are not committing a crime. "We cannot criminalize people who are working, people who are contributing to our economy, and contributing to the nation," he said.

Protesters say the United States needs workers and the unemployed in Mexico need jobs. One counter-protester said there are laws already in place for people to enter the country. "I'd like to see immigration laws being enforced," he said.

President Bush used his weekly radio address to repeat his call for a guest worker program that would provide legal status, but not citizenship, for some illegal immigrants in the United States.

Some in his own Republican party, including Senate majority leader Bill Frist, say border security and enforcement should shape the debate over immigration reform. Some other Republicans, and many Democrats, support some form of guest worker program. The Senate will begin debating the issue next week.

block a bill: 阻止法案通过 

felony: one of several grave crimes （重罪）