A US federal judge yesterday 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 penalise 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, Judge 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. Judge 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 per cent 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 today amid fears that the new law will lead to racial profiling.