Arizona Governor Jan Brewer said she would file an appeal to reinstate the provisions, which had popular support but were opposed by President Barack Obama and immigration and human rights groups.

U.S. District Judge Susan Bolton blocked several provisions including one that required a police officer to determine the immigration status of a person detained or arrested, if the officer believed the person was not in the country legally.

The judge also stayed provisions requiring immigrants to carry their papers at all times and making it illegal for people without immigration papers to seek work in public places.

The Republican-controlled Arizona Legislature passed the law three months ago in an effort to drive nearly half-a-million illegal immigrants out of the border state and stem the flow of human and drug smugglers over the frontier.

The U.S. Justice Department had argued that provisions of the law, which goes into effect on Thursday, encroached on federal authority over immigration policy and enforcement.

In her 36-page decision, Bolton agreed. She wrote: "The court ... finds that the United States is likely to suffer irreparable harm if the court does not preliminarily enjoin enforcement of these Sections of (the law) and that the balance of equities tips in the United States' favor considering the public interest."

COULD GO TO SUPREME COURT

Governor Brewer said the state would quickly file an appeal.

"We will take a close look at every single element Judge (Susan) Bolton removed from the law, and we will soon file an expedited appeal at the United States Court of Appeals for the 9th Circuit," she said in a statement.

Arizona can appeal ultimately to the U.S. Supreme Court. In that case, it could become a long, protracted battle.

About three dozen Hispanic activists attending an open-air mass outside the state capitol in central Phoenix jumped up, hugged and wept as news of Bolton's ruling broke.

"I think that our efforts have paid off," said Dulce Matuz, an undocumented college graduate who has been living in the desert state without papers for a decade, adding that activists would carry on fighting to overturn the rest of the law.

State Senator Russell Pearce, the architect of the law, said he was "very disappointed" by the judge's ruling.

"What she did was decide to insert some opinion into the law rather than rule on what is the law of the land, and that's not right," Pearce told Reuters. "But we will win on appeal."

"WIND OUT OF SAILS"

Peter Spiro, a law professor at Philadelphia's Temple University and a former attorney in the State Department, said he was not surprised the more controversial provisions were stopped from being put into effect.

"I expect those provisions will never go into effect, though this is only a preliminary order," Spiro said.

"I also think this will take the wind out of the sails of anti-immigration efforts on the state level, though it will probably intensify such efforts at the federal level," he said.

The Arizona law is the toughest anti-immigration measure in any U.S. state but it is inspiring copycat efforts in at least 20 other states. There are an estimated 10.8 million illegal immigrants in the country.

Polls show the Arizona law is backed by a solid majority of Americans and by 65 percent of the state's voters.

It is a sensitive political issue ahead of congressional elections in November in which Obama's Democrats are battling to retain control of Congress amid popular anger at over high unemployment and soaring deficits.

Opponents say the law would lead to harassment of Hispanic or Hispanic-looking Americans. Thousands were headed to Phoenix for protests on Thursday and street rallies were planned across the country from California to Washington, D.C.

'DISCRIMINATION IS ILLEGAL'

Chris Newman, general counsel for the National Day Laborer Organizing Network, said the ruling was expected and that protests and rallies would go ahead as planned on Thursday.

Police across the desert state, which is the principal corridor for human and drug smugglers entering the United States from Mexico, have been preparing to implement the law.

The state's 15,000 law enforcement officers have had training on how to implement it and identify people they suspect are unlawfully in the state without resorting to racial profiling, Governor Brewer had said on Tuesday.

Brewer said she felt "very comfortable that everybody is being well trained," and officers using racial criteria to implement the law would be punished.

"Racial discrimination is illegal. It's illegal in the United States, it's illegal in Arizona, it has been and it will continue to be," Brewer said on CNN's Larry King Live show.

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