Up to 500,000 people marched through Dallas on Sunday waving US flags and shouting "Si se puede" - Spanish for "Yes, we can!"
They included families pushing strollers with their children and ice cream vendors who placed American flags on their carts. Many wore white clothing to symbolise peace.
"My mom was born in Mexico. She's been here ever since she was 16, and I'm here for her because I don't believe (in) the bill on immigration," said protester Ambrosio Garcia, 25.
"Immigrants should not be criticised or considered illegal felons."
The Dallas march was among several demonstrations that drew thousands of protesters in New Mexico, Minnesota, Michigan, Alabama, Utah, Oregon and California.
The demonstrations came in advance of nationwide protests set weeks ago for Monday, a signal that what began as a series of disparate events - attracting tens and even hundreds of thousands of people - has become more co-ordinated.
Many groups had been preparing to rally since December, when the House of Representatives passed a bill to build more walls along the US-Mexico border; make criminals of people who helped undocumented immigrants; and make it a criminal offence, rather than a civil infraction, to be in the country illegally.
But the top Republican in the House said on Sunday that he remained opposed to a guest worker programme for illegal immigrants and supported the December bill.
"You can't begin to talk about a guest worker bill until you secure the borders," said John Boehner on ABC television.
Walls along the border with
Mexico are being envisaged
Otherwise, he said, "we're going to have an endless parade of illegal immigrants here in our country".
A bipartisan compromise that would have put millions of illegal immigrants on the road to citizenship broke down in the Senate on Friday amid bickering between Democrats and Republicans over possible amendments.
The collapse of the bill raised doubts over whether Congress can pass the comprehensive immigration reform that George Bush, the president, has called for before November mid-term elections.
The House bill is tougher than the proposed Senate version as it defines the illegal immigrants as felons, or serious criminals.
If senators pass a bill, legislators from both sides of Congress would have to work out their differences for a final bill. Boehner said that was possible.
Boehner stopped short of saying congressional efforts to reform immigration had failed, calling on the Senate to pass a bill when lawmakers return from recess in two weeks.
Still, he said, allowing illegal immigrants to stay and work "sounds too much like amnesty for most Americans".
The issue has divided conservatives, some of whom are anxious to court the Hispanic vote and support Bush.
Others worry that allowing in undocumented immigrants, mostly from Mexico, could harm their election efforts.
"You can't begin to talk about a guest worker bill until you secure the borders [otherwise] we're going to have an endless parade of illegal immigrants here in our country"
House Majority Leader John Boehner
"Everybody agrees there's an enormous problem, everybody agrees with the border security lines and there's general agreement that we have to craft a compromise," Arlen Specter, the Senate Judiciary Committee chairman, told Fox News television.
Luis Gutierrez, a House member from Illinois and the chairman of the Democratic Caucus Immigration Task Force, said any bill must deal with the illegal immigrants already in America because too many do tough agricultural and other essential work.
"The only sane, sensible, compassionate thing to do is to integrate them fully into the fabric of our society," he told NBC television.
Henry Bonilla, a Texas lawmaker, said most conservatives would eventually accept integration. "A lot of us want to support a guest-worker plan down the road, but first and foremost, we have to secure the border," he told NBC.