Many Hispanic groups urged their members to forgo work, school and shopping on Monday to press politicians to allow an estimated 12 million mostly Latino undocumented workers to stay in the US.

"We have to make our presence felt through our absence," leaders of a network representing about 40 million Hispanics said in a statement.

Monday is a normal work day in the United States, where Labour Day is celebrated in September instead of on May 1.

In recent months, the issue of immigration has come to dominate much of the US political agenda, and is expected to be a key issue in the mid-term congressional elections later this year

'Great American boycott'

In Los Angeles, officials are bracing for huge crowds to take to the streets on Monday.

George Gascon, the city's assistant police chief, said that as many as 500,000 people could take part in what is collectively called Un Dia Sin Immigrantes – A Day Without Immigrants.

Tens of thousands have taken to
the streets before in protest

Nativo Lopez, president of the Mexican-American Political  Association, said: "We've unequivocally called on all families to participate in the Great American Boycott and the marches - and that translates into not going to work, not going to school, not shopping and not selling."

However, city and school officials and the We Are America coalition, which includes the Roman Catholic church, are encouraging people to go to school and work and then join the demonstrations later in the day.

The division over the way the protests should be carried out is as evident among political leaders as it is among unions.

Bush opposed

George Bush, the president, has criticised the boycott and a measure to have a Spanish version of the US national anthem, urging immigrants instead to learn English so they can sing the original version of the song.

California's Republican Governor Arnold Schwarzenegger also denounced the boycott, although the state's Democrat-dominated Senate approved a resolution supporting it.

Though the economic impact of the day is hard to predict, some commentators are likening the social impact to the dawning of the US civil rights movement in the 1960s.

Walkout

Already, food-processing giants such as Tyson Food Inc and Cargill Foods have announced they are closing some of their plants in anticipation of a walkout.

Both said they understood the sentiments behind the protest.

"In California, the world's fifth-largest economy, one in every four residents was born elsewhere, so it is important to realise that our economy has come to depend on immigrants"

Gloria Romero,
Democratic state senator

Angel Luevano, spokesman for the League of United Latin American Citizens, said: "It's no surprise that food companies are taking a position favouring workers, given that many of them employ immigrants, most of them Hispanic and many of them illegal."

Wal-Mart, which employs more than 1.3 million people, has not yet announced what it will do if a sizable number of its Hispanic employees take the day off.

The US Hispanic Chamber of Commerce, however, said that workers must first get their bosses' consent to be absent from work on Monday, in order to protect themselves from being fired.

None the less, it voiced support for the "just reform" the immigrants are seeking.

Businesses' backing

The stand taken by the chamber - representing nearly two million Hispanic-owned businesses that bring in $300 billion  in sales each year - was echoed by the National Council of Chain Restaurants, which represents 40 different chains with about 2.8 million workers.

Protesters oppose plans to build
extra fencing on the border

Leaders from unions within the AFL-CIO, a labour umbrella group of more than nine million workers, said they supported the strike and pledged to join the demonstrations.

But the United Farm Workers' Union, although it supports immigration reform, urged members to take part in the rally after work to avoid being fired.

Meanwhile, a group of lawyers in Los Angeles announced that they would be outside the city's immigration office on May 5 to volunteer their services to anyone who was penalised for taking part in the demonstration.

"In California, the world's fifth-largest economy, one in every four residents was born elsewhere, so it is important to realise that our economy has come to depend on immigrants," Gloria Romero, the Democratic state senator who introduced the resolution supporting the boycott, said on Friday.

Unions, civil rights groups, schools and churches oppose a bill in the US congress which would make illegal immigration a full criminal offence.

The protesters want Congress to approve a provision allowing undocumented workers to remain in the country.

They also oppose plans to construct extra security fencing on the Mexican border.