WASHINGTON - The US government filed suit Tuesday against Arizona in a bid to block the state's controversial immigration law which President Barack Obama's administration has slammed.

A Justice Department statement said the state law hampered the authority of the Obama administration to enforce national immigration policy.

Federal laws do not permit the development of a "patchwork of state and local immigration policies," it said.

"Setting immigration policy and enforcing immigration laws is a national responsibility," US Attorney General Eric Holder said in the statement.

Arizona, the Justice Department said, "crossed a constitutional line."

The Arizona law, due to take effect on July 29, makes it a crime to be in the state, which borders Mexico, without proper immigration papers.

It also requires local police, who are not federal agents responsible for immigration matters, to determine if people are in the country legally. Many critics charge that will fuel ethnic profiling.

US officials said they took the action after consulting with various law enforcement departments, chiefs of police, civil rights groups and other local officials.

Many said they were concerned the law would make victims of crimes or witnesses "less likely to contact or cooperate with law enforcement" if they did not possess proper legal papers. Critics also charge the law will prod local police to single out for questioning people they think appear not to be American.

Officials in Arizona argue they have been overrun by illegal immigrants leading to a spike in the crime rate and straining state resources. They say the measure was necessary only because of lax federal government enforcement of the southern US border.

"Arizonans are understandably frustrated with illegal immigration, and the federal government has a responsibility to comprehensively address those concerns," Holder said.

However, he added, "diverting federal resources away from dangerous aliens such as terrorism suspects and aliens with criminal records will impact the entire country's safety."

Meanwhile, Arizona's two US senators issued a joint statement slamming the lawsuit.

"The American people must wonder whether the Obama administration is really committed to securing the border when it sues a state that is simply trying to protect its people by enforcing immigration law," Republicans John McCain and Jon Kyl said in a joint statement.

US voters broadly back tough enforcement of laws. But traditionally, Americans -- most descended from immigrants or immigrants themselves -- were willing to look the other way in cases of non-compliance with immigration laws. That old truism appears to have faded since the September 11 terror strikes.

More than 60 percent of the US population supports Arizona's new immigration law, according to a recent opinion survey.

About 30 percent of Arizona's population of 6.6 million are Hispanic, one third of whom are foreign born, including 460,000 illegal immigrants.

Department of Homeland Security Secretary Janet Napolitano said in the statement that her staff was "actively working with members of Congress from both parties to comprehensively reform our immigration system at the federal level because this challenge cannot be solved by a patchwork of inconsistent state laws, of which (Arizona's) is one."

The American Civil Liberties Union was among groups praising the administration.

"The administration's lawsuit is a cannon shot across the bow of other states that may be tempted to follow Arizona's misguided approach," said

Lucas Guttentag, director of the ACLU Immigrants' Rights Project.

In Mexico, which has been highly critical of the new measure, the Foreign Ministry said Tuesday that the government of President Felipe Calderon "welcomes efforts to stop the Arizona law and declare it unconstitutional."

Outside the White House, activists opposing the Arizona law launched a 24-hour vigil demanding that it be struck down. "Arizona is here to ask the government to veto the law," said activist Rosa Maria Soto, holding a crucifix, and calling for "adoption of fair and comprehensive immigration reform."

Obama on July 1 renewed his called for Republicans to back immigration reform to bring 11 million illegal immigrants out of the shadows, but his political foes quickly accused him of "political pandering" four months ahead of key legislative elections.