A US judge has blocked key parts of Arizona's tough new immigration law hours before it was to take effect, handing a victory to the Obama administration as it tries to take control of the issue.
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.
"This fight is far from over," Ms Brewer said, adding that "at the end of what is certain to be a long legal struggle, Arizona will prevail in its right to protect our citizens".
The Republican-controlled state legislature passed the law three months ago to try to drive nearly half a million illegal immigrants out of Arizona and stem the flow of human and drug smugglers over the border from Mexico.
The provisions blocked by US District Judge Susan Bolton included 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.
Judge Bolton also halted provisions requiring immigrants to carry their papers at all times and making it illegal for people without proper documents to tout for work in public places.
Immigration as an issue has festered in US politics for years and attempts to overhaul the system have failed, most recently in 2007 when Republicans torpedoed reforms pushed by former president George W. Bush.
The ruling is a significant victory for Mr Obama, who wants to break the deadlock with Republicans to pass a comprehensive immigration law tightening border security and giving millions of illegal immigrants a shot at legal status - an already difficult task before November's congressional elections.
There are an estimated 10.8 million illegal immigrants in the United States, a country of more than 300 million people.
The Justice Department had argued provisions of the Arizona law, which goes into effect today, encroached on federal authority over immigration policy and enforcement.
In her 36-page decision, Judge Bolton agreed, finding "the United States is likely to suffer irreparable harm" if her court did not block the selected parts of the law.
"The number of requests that will emanate from Arizona as a result of determining the status of every arrestee is likely to impermissibly burden federal resources and redirect federal agencies away from the priorities they have established," she said.
Judge Bolton kept some parts of the law, including provisions making it illegal for drivers to pick up day labourers off the street and to transport or harbor an illegal immigrant.
Ms Brewer said her state "will soon file an expedited appeal" with the US Court of Appeals for the Ninth Circuit.
Arizona can appeal ultimately to the US Supreme Court. In that case, it could embroil the cash-strapped desert state in a protracted and costly legal battle.
"There are no winners here," said Representative Ann Kirkpatrick, an Arizona Democrat who faces a tough battle for re-election in November. "No matter what the courts ultimately decide, we will still have wasted millions of dollars and our borders will still not be secure."
John McCain and Jon Kyl, Arizona's Republican US senators, said they were "deeply disappointed" by the ruling and took aim at Obama for failing to provide resources to secure the state's porous border with Mexico.
"Instead of wasting taxpayer resources filing a lawsuit against Arizona ... the Obama administration should have focused its efforts on working with Congress to provide the necessary resources to support the state in its efforts to act where the federal government has failed," their statement said.
Mr McCain, a Republican moderate, faces a tough challenge in the party's state primary election from former congressman JD Hayworth, a conservative who is hawkish on border issues.
About three dozen Hispanic activists at an open-air mass outside the state capitol in central Phoenix jumped up, hugged and wept as news of Judge Bolton's ruling broke.
"I think that our efforts have paid off," said Dulce Matuz, a college graduate who has lived in Arizona without papers for a decade, adding activists would carry on fighting to overturn the rest of the law.
The Mexican government hailed the ruling as a "step in the right direction." Around 100 activists cheered and chanted "Yes we can" and "No to xenophobia" as news of the ruling reached a rally outside the US embassy in Mexico City.
Reuters