More than 10,000 immigrants and their supporters clogged the streets in front of Los Angeles City Hall on Saturday to protest against a proposed law they see as punitive to undocumented workers.
Javier Rodriguez, of the March 25 Coalition, which organised the event, said: "We have got to stop the approval of anti-immigrant reforms, demand a migration reform that is humane and fair, and not racist."
The coalition represents dozens of pro-immigrant groups, especially migrants from Latin America. But the protesters included farmers, labour unions, religious groups and students.
Rodriguez billed the event the "largest Hispanic protest march in US history".
On Friday, as many as 15,000 marched in Phoenix in a similar demonstration.
The immigration reform bill to be debated in Congress next week targets illegal immigrants, who number about 11.5 million according to the Pew Hispanic Centre and account for 24% of farm workers, 17% of cleaners and 14% of construction workers.
The law would make all undocumented immigrants criminals, and require all employers to verify the immigration status of their employees.
Bush weighed in on the immigration debate in his weekly radio address as the Senate was days away from taking up legislation on the subject.
The president favours including a temporary work visa in broad legislation that would also bolster border enforcement.
But some Republicans view the guest-worker plan as a back-door amnesty for illegal immigrants, and prefer an approach that focuses solely on reinforcing border security and cracking down on illegal immigration.
Bush: Securing borders is a top
priority of immigration reform
George Bush, who this week urged all sides to tone down their rhetoric in the emotional debate, said securing borders was a top priority of immigration reform but invoked the country's history as "a nation of immigrants" to argue for a balanced approach.
"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," the president said.
Arlen Specter, Senate Judiciary Committee chairman, wants to finish work on the legislation that includes a temporary worker programme and would give undocumented aliens an opportunity to legalise their status.
The Pennsylvania Republican is working against a deadline set by Bill Frist, the Senate Majority Leader, and potential 2008 presidential candidate.
Frist, a Tennessee Republican, plans to bring to the Senate floor his own border-enforcement immigration bill if the Judiciary Committee fails to pass legislation.
Looming poll issue
The panel is to meet on Monday in hopes of rushing legislation to the floor before Frist brings up his own bill.
Immigration is looming as an issue in the November mid-term elections in which Republicans are seeking to hold on to their majorities in both houses of Congress.
But the politics of border security have created competing pressures for Republicans.
A guest-worker plan would offer
respite to illegal immigrants
Bush views the guest-worker programme as a way of courting Hispanic voters in states such as Arizona, New Mexico and Florida.
But some conservative Republicans are focusing on enforcement as constituents vent frustration at what they see as a strain on schools, hospitals and other local resources from illegal immigration.
Although the guest-worker plan would offer illegal immigrants a chance to register and work in the US for up to six years, Bush rejected any description of it as an amnesty.
"I believe that granting amnesty would be unfair, because it would allow those who break the law to jump ahead of people who play by the rules and wait in the citizenship line," Bush said.