A federal judge Wednesday blocked the most controversial parts of Arizona's new immigration law, barring police from checking the immigrant status of suspected criminals.

The ruling came hours before the new law had been due to go into effect, handing temporary victory to civil rights groups and the Obama administration which has challenged the legislation.

For the first time in the United States -- a nation built on generations of immigrants -- the law would make illegal immigration a crime and penalize anybody helping or giving work to undocumented workers.

US District Court Judge Susan Bolton is currently hearing seven suits lodged against the legislation, signed into law by Republican governor Jan Brewer.

White House lawyers have argued immigration policy is exclusively the government's responsibility and that state laws cannot trump federal rules or the US constitution.

In her ruling, Bolton said the US administration "is likely to succeed" in its argument and issued a preliminary injunction suspending the section of the Arizona law requiring police officers to check the immigrant status of any person they have stopped for a violation.

She also blocked a provision making it a crime to fail to apply for or carry proper papers, and a third section making it a crime for illegal immigrants to apply for or perform any work.

Bolton ruled that she was issuing the injunction against the most controversial elements of the law because otherwise "the United States is likely to suffer irreparable harm."

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

But thousands of people have been gathering in Phoenix, the capital of the southwestern state, preparing for a protest march Thursday amid fears that the new law will lead to racial profiling.

One third of the 6.6 million population of Arizona, which borders Mexico, is foreign born and an estimated 460,000 are illegal immigrants.

"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.

But Arizona officials say 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.

Roman Catholic Church leaders, Mexico and several other countries however have objected to the 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.