PHOENIX (AP) — A federal judge dealt a serious blow to Arizona’s toughest-in-the-nation immigration law on Wednesday when she put most of the crackdown on hold just hours before it was to take effect.

The ruling by U.S. District Judge Susan Bolton sets up a lengthy legal battle that may not be decided until the Supreme Court weighs in.

The Obama administration, one of the litigants in the case, opposes the state legislation on grounds it usurps federal authority over immigration policy. Republican Gov. Jan Brewer said the state likely will appeal the ruling and seek to get the judge’s order overturned.

For now, opponents of the law have prevailed: The provisions that most angered opponents will not take effect, including sections that required officers to check a person’s immigration status while enforcing other laws.

The judge also delayed parts of the law that required immigrants to carry their papers at all times and made it illegal for undocumented workers to solicit employment in public places — a move aimed at day laborers. In addition, the judge blocked officers from making warrantless arrests of suspected illegal immigrants.

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“Requiring Arizona law enforcement officials and agencies to determine the immigration status of every person who is arrested burdens lawfully present aliens because their liberty will be restricted while their status is checked,” Bolton said in her decision.

She said the controversial sections should be put on hold until the courts resolve the issues. Other provisions of the law, many of them slight revisions to existing Arizona immigration statute, will go into effect at 12:01 a.m. Thursday.

The law was signed by Brewer in April and immediately revived the national debate on immigration, making it a hot-button issue in the midterm elections. The law has inspired similar action elsewhere, prompted a boycott against Arizona and led an unknown number of illegal immigrants to leave the state.

Lawyers for the state contend the law was a constitutionally sound attempt by Arizona to assist federal immigration agents and lessen border woes such as the heavy costs for educating, jailing and providing health care for illegal immigrants. Arizona is the busiest gateway into the country for illegal immigrants, and the state’s border with Mexico is awash in drugs and smugglers that authorities badly want to stop.

“It’s a temporary bump in the road, we will move forward, and I’m sure that after consultation with our counsel we will appeal,” Brewer told The Associated Press. “The bottom line is we’ve known all along that it is the responsibility of the feds and they haven’t done their job so we were going to help them do that.”

The ruling came just as police were making last-minute preparations to begin enforcement of the law and protesters were planning large demonstrations against the measure. At least one group planned to block access to federal offices, daring officers to ask them about their immigration status.

In a sign of the international interest in the law, about 100 protesters in Mexico City who had gathered in front of the U.S. Embassy broke into cheers when speakers told them about the federal judge’s ruling. The demonstrators had been monitoring the news on a laptop computer on the stage.

The crowd clapped and started chanting, “Migrants, hang on, the people are rising up!”

Gisela and Eduardo Diaz went to the Mexican consulate in Phoenix on Wednesday seeking advice because they were worried about what would happen to their 3-year-old granddaughter if they were pulled over by police and taken to a detention center.

“I knew the judge would say that part of the law was just not right,” said Diaz, a 50-year-old from Mexico City who came to Arizona on a since-expired tourist visa in 1989. “It’s the part we were worried about. This is a big relief for us.”

Opponents argued the law would lead to racial profiling, conflict with federal immigration law and distract local police from fighting more serious crimes. The U.S. Justice Department, civil rights groups and a Phoenix police officer had asked the judge for an injunction to prevent the law from being enforced.

“There is a substantial likelihood that officers will wrongfully arrest legal resident aliens under the new (law),” Bolton ruled.

Federal authorities have argued that letting the Arizona law stand would create a patchwork of immigration laws nationwide that would needlessly complicate the foreign relations. Federal lawyers said the law is disrupting U.S. relations with Mexico and other countries and would burden the agency that responds to immigration-status inquiries.

The core of the government’s case is that federal immigration law trumps state law — an issue known as “pre-emption” in legal circles.

The judge pointed out five portions of the law where she believed the federal government would likely succeed on its claims that U.S. law supersedes state law.

“Even though Arizona’s interests may be consistent with those of the federal government, it is not in the public interest for Arizona to enforce pre-empted laws,” Bolton wrote.

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Associated Press Writers Michelle Price, Bob Christie and Paul Davenport contributed to this report in Phoenix; Olga R. Rodriguez contributed from Mexico City.

Copyright 2010 The Associated Press.