Thousands are expected to march Thursday when an Arizona law making illegal immigration a crime goes into effect over government objections and amid fears it will lead to ethnic profiling.

Passed by the Arizona state legislature in April, the law has been challenged by the federal government with a possibility that a judge might delay its implementation.

But that will not placate its opponents.

"We are waiting for the court to decide, but even if it issues a temporary injunction... we're still going ahead with our protests, because 21 other states want to follow Arizona's footsteps with racist laws" of their own, Paulina Gonzalez, spokeswoman for the "We Are All Arizona" group told AFP.

The protest movement has come alive in Arizona, where one third of the 6.6 million population is foreign born and an estimated 460,000 are illegal immigrants.

The eyes of the entire nation are fixed on this southwestern state, as the issue of immigration has grown in national scope both due to the recent economic downturn and the upcoming November legislative elections.

"Thursday will be our national civil disobedience day, when we'll stand up to a racist, discriminatory and hypocritical measure that targets the very people who work for those who speak against undocumented workers," National Day Laborer Organizing Network director, Pablo Alvarado, told AFP.

Signed into law by Governor Jan Brewer on April 23, law SB1070 includes a provision especially rankling for civil rights group that allows police to ask for documents verifying a person's immigration status while checking for any violation, such as during a traffic stop.

For the first time in the United States, the law makes illegal immigration a crime and penalizes anybody helping or giving work to undocumented workers.

Civil rights leaders fear the law will lead to widespread ethnic profiling.

Last week, federal judge Susan Bolton heard arguments for and against the law.

White House lawyers argued in a packed court room that immigration policy is exclusively the government's responsibility and that state laws cannot trump federal rules or the US Constitution.

But Arizona officials said they have been overrun by illegal immigrants who fueled a spike in the state's crime rate and put a strain on state resources. They said the measure was necessary only because of lax federal government enforcement of the southern US border.

During the hearing, Judge Bolton commented that the Arizona law was ambiguous and "awkward" in its wording and that she was doubtful it could be duly enforced.

The protest movement in Arizona will be bolstered by demonstrators from around the United States.

In Los Angeles "between 10 and 12 buses will fill up with all kinds of different people black, Asian, Hispanic and many white people from across the country. They'll travel six hours to Phoenix to march against the law," said Alvarado.

"Nobody on the buses will travel with an ID," he added, in open defiance of the new law that calls for police to ask for documents during routine checks.

Recent opinion polls found more than 60 percent of the US population supporting the Arizona immigration law.

On Tuesday, a Gallup poll found that 45 percent of Americans believe immigration should decrease in the country, against 17 percent who said it should increase, and 34 percent that it should stay at current levels.

The July 8-11 telephone survey on immigration of 1,020 adults showed a narrowing of Americans' views on the subject since last year, when 50 percent wanted immigration to decrease and 14 percent favored an increase.

Since the September 11, 2001 terrorist attacks on the United States, the opinion tracker said, Americans have taken a tougher stance on immigration.

Roman Catholic Church leaders, Mexico and several other countries have objected to the Arizona law, including lawmakers from Latin American countries who last week attended a world conference of legislative leaders in Geneva.

"We regret that this law has a racist and xenophobic spirit that goes against immigration in general and illegal immigrants in particular," said a statement by parliament leaders of Mexico, Ecuador, Uruguay, Panama, Bolivia, Guatemala, Cuba and Chile.