About half a million activists have rallied in Los Angeles in the US state of California to protest against plans to criminalise undocumented workers.
The protest is one of the largest in the city for many years
Organiser Javier Rodriguez said demonstrators wanted an immigration system that was humane and not racist.
The House of Representatives has passed a bill that would make it a felony for immigrants to be in the US illegally.
The Senate is set to discuss the bill on Tuesday. President George W Bush has proposed a guest-worker plan.
He is proposing to allow foreigners to remain in the US for a set period of time to carry out specific jobs, but his Republican Party is divided over the issue.
The president this week urged all sides of the debate to tone down their rhetoric.
He said securing borders was a top priority but he also invoked the country's history as "a nation of immigrants" to argue for a balanced approach.
The new bill would also impose new penalties on employers who hire illegal immigrants and allow for the erecting of fences along a third of the US-Mexican border.
The proposals have angered many Hispanic-Americans, a key voting bloc in November's mid-term elections.
Immigrants are said to do many of the jobs Americans do not want to
Mr Rodriguez, of the March 25 Coalition, said he wanted to stop "the approval of anti-immigrant reforms" and demand "migration reform that is humane and fair, and not racist".
Protester Lionel Vanegas told the Reuters news agency that the bill was wrong "because this is a country for everybody who wants to live a better life and this is a free world".
It is estimated that 11.5 million people are living in the US illegally. Many of them work in the agricultural sector and the construction and service industries.
The debate on the issue has intensified in the past week, with protests against the bill on Friday in the cities of Milwaukee and Phoenix.
On the other side of the debate, supporters of tighter border controls are planning to take to the streets of Washington and Boston on Monday.