LOS ANGELES, March 25 -- They surprised the police, and maybe themselves, their T-shirts turning block after block of downtown Los Angeles streets white in a demonstration so massive that few causes in recent U.S. history could have matched it.

Police said more than 500,000 people marched Saturday to protest a proposed federal crackdown on illegal immigration. Wearing white as a sign of peace -- and waving flags from the United States, Mexico, Guatemala and other countries -- they came to show that illegal immigrants are already part of the American fabric, and want the chance to be legal, law-abiding citizens.

Police used helicopters to come up with the crowd estimate. "I've been on the force 38 years, and I've never seen a rally this big," said Cmdr. Louis Gray Jr., incident commander for the rally.

In Charlotte, between 5,000 and 7,000 people gathered Saturday carrying signs with slogans such as "Am I Not a Human Being?" In Sacramento, more than 4,000 people protested during a march honoring the late farm labor leader Cesar Chavez. In Denver, more than 50,000 people protested downtown, according to police who had expected only a few thousand.

Phoenix was similarly surprised Friday when an estimated 20,000 people gathered for one of the biggest demonstrations in city history, and more than 10,000 marched in Milwaukee on Thursday.

The demonstrators oppose legislation passed by the U.S. House that would make it a felony to be in the United States illegally. It would also impose new penalties on employers who hire illegal immigrants, require churches to check the legal status of parishioners before helping them, and order the construction of fences along one-third of the U.S.-Mexican border.

"I think it's just inhumane. . . . Everybody deserves the right to a better life," said Elger Aloy of Riverside, 26, who brought his infant son to the Los Angeles march.

Many of the demonstrators who weren't immigrants said they have relatives who are. "My mom came from Mexico. She had to cross the river, and thank God she did," said David Gonzalez, 22. He rejected assertions by advocates of the legislation that it would help protect the nation from terrorism. He said it would hurt Hispanics the most.

The Senate is to begin debating immigration proposals Tuesday.

President Bush is pushing for a guest-worker program that could provide temporary legal status to some of the estimated 12 million illegal immigrants. Many of his fellow Republicans are taking a more restrictive stance.