LOS ANGELES, May 1 (Xinhua) -- Protesters began arriving in major streets in downtown Los Angeles on Monday to prepare for immigration marches, expected to be the largest in the city's history.

    The marches, scheduled to begin at noon, were part of large-scale nation-wide demonstrations named "American Boycott" and "A Day Without Immigrants", and were expected to draw more than 500,000 people.

    Police said they were bracing for anything unexpected but did not think violence and conflict would occur. As a precaution, police were preparing to close streets downtown.

    Thousands of undocumented workers and their supporters had flexed their political and economic muscles these days, calling onworkers to stay away from work and students to quit class on May Day in the hope of overhauling immigration policy.

    Demonstrations, rallies and boycotts were planned from New Yorkto Chicago and across the South and Southwest. The earliest marches began this morning in Florida. Monday's marches would be the third major demonstrations in about six weeks.

    Monday'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 weekwarned motorists in Spanish: "No work, no school, no buying, no selling."

    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 fourpork 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.

    The House passed a bill last December which would criminalize undocumented immigrants and those who help them, sparking nation-wide protests. The bill would also beef up enforcement at the border, including adding 700 miles of fencing.

    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 George W. Bush supports a guest-worker program and has praised a Senate approach, backed by top Republicans and Democrats. That approach would lead to citizenship for those in the United States illegally, while enforcing border controls.

    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, according to the Los Angeles Times. Enditem