Thousands of undocumented workers and their supporters this morning flexed their political and economic muscles in the hope of overhauling immigration policy.

Demonstrations, rallies and boycotts were planned from New York to Chicago to California and across the South and Southwest, as immigration rights activists staged the third major day of protest in about six weeks. The earliest marches began this morning in Florida.

In Los Angeles, protesters began arriving as police prepared to close streets downtown in preparation for the first of two marches. The downtown march was scheduled for noon along Broadway from Olympic Boulevard to City Hall. Officials expect many of those participants to then head to MacArthur Park for a second march planned for the afternoon down Wilshire Boulevard to La Brea Avenue.

Wilshire was to be closed to traffic beginning at 2 p.m. Other demonstrations were planned in Pasadena, Pomona, Santa Ana, Fullerton, Riverside, San Diego, Oxnard, Huntington Park, Long Beach and San Bernardino. Washington has been debating its first major revision of immigration policy in two decades. There is bipartisan support for toughening border security, but the key issues of how to deal with the estimated 11 million to 12 million undocumented workers in the United States and a guest worker plan has opened fissures in both parties.

The issue has also created unusual political alliances. Major industries, including parts of agriculture, apparel and tourism, have joined with labor unions and human rights activists to support changes. Conservatives in both the Democrat and Republican parties have opposed guest-worker programs and paths for citizenship for those illegally in the United States. President Bush supports a guest-worker program and has praised a Senate approach, backed by top Republicans and Democrats. The House has passed a harsher plan that would make it a felony to be an illegal immigrant or to assist them.

Today's protests represent an evolution of ideas since an estimated 500,000 people rallied on March 25 in front of Los Angeles City Hall to protest the House's Draconian plan. Relying on the Latino media, especially radio personalities, and high-tech communications, the rally caught the Anglo establishment flat-footed. In the following days, schools, particularly in Southern California, were forced to close as students repeatedly walked out to support immigration rights. On April 10, hundreds of thousands of people rallied across the country for broader immigrant rights.

Today's demonstrations were based on the idea of making the Latino presence felt politically, but also economically by boycotting businesses. It was being organized by dozens of groups under the slogan of Un Dia Sin Inmigrantes — A Day Without Immigrants. A hand-scrawled sign on the Pasadena Freeway last week warned motorists in Spanish: "No work, no school, no buying, no selling." As far away as Mexico, activists echoed the boycott call, urging people to forgo U.S. companies and even to skip American fast food outlets.

Some major U.S. companies, dependent on immigrant labor, announced that they would be closed for the day. Tyson Foods, the world's largest meat producer, shut nine U.S. beef plants and four pork plants. Cargill, the second-largest meatpacker, also closed some plants. Six of 14 Perdue Farms plants were closed and Gallo Wines in Sonoma, Calif., gave its 150 workers the day off. Still, the long-term economic impact was likely to be minimal — though still painful — for small businesses and low-paid workers. Except for meals, purchases given up today can be made up tomorrow. Many businesses, including service and manufacturing industries, had employees work through the weekend to get ahead so that today's closings would not be disastrous.

Today's boycotts also split the Latino community. Those calling for the boycotts tended to be from non-traditional groups, while leaders of established institutions urged students to stay in class and were neutral at best about whether workers should take the day off for the morning protest. That split was clearly illustrated in Southern California. Mayor Antonio Villaraigosa, the first Latino in more than 130 years to have the city's top job, and Cardinal Roger M. Mahony on Sunday repeated their pleas that students not take part in the day's activities until after school. Villaraigosa also warned motorists to prepare for gridlock in the affected areas.

"Democracy is not always convenient, and it's unavoidable that Angelenos will be inconvenienced by these protests," he said. A spokesman for Villaraigosa said the mayor has no plans to take part in the marches but will monitor them from City Hall. He was scheduled to fly to Texas to urge National Football League officials to bring a team to Los Angeles.

Demonstrators today were expected to again wear white as a gesture of peace and to wave U.S. flags in the hopes of decreasing any backlash. Organizers have urged protesters to avoid carrying Mexican flags or singing the national anthem in any language other than English. A Spanish version of the anthem, released last week, sparked complaints, including from President Bush. Over the weekend, some groups, which call for tougher enforcement against illegal immigrants, held their own demonstrations along the U.S.-Mexican border and promised more protests. About 200 volunteers organized by the Minuteman Civil Defense Corps of California worked on a 6-foot barbed-wire fence along a quarter-mile stretch of rugged terrain near the U.S.-Mexico border about 50 miles east of San Diego.

The U.S. government has also vowed to crack down on employers who hire illegal immigrants. And there have been well-publicized raids by immigration authorities, rounding up hundreds of immigrants with criminal backgrounds in Florida and the Midwest. Still, most polls show that Americans favor changes in immigration policy by better than a ratio of 2 to 1 and that Californians are even more supportive. Americans tell pollsters that they would like to see a guest worker program and some sort of plan that would lead to citizenship for those in the United States.