Arizona Gov. Jan Brewer on Friday signed the toughest law against illegal immigration in the country, shrugging aside warnings from religious and civil rights leaders - and President Obama - that it would lead to widespread racial profiling.

Hours after Obama denounced the measure as "misguided," Brewer held a signing ceremony for the bill, which makes it a crime to be in the state illegally and requires police to check suspects for immigration paperwork.

Obama signaled that a legal showdown might be possible and that his administration would "examine the civil rights and other implications" of the law. Department of Justice officials said they "were reviewing the bill" but declined to discuss the legislation further. Immigrant rights groups have vowed a court fight, arguing that regulating immigration is a federal matter.

Brewer, at an afternoon news conference in Phoenix, cast the law in terms of public safety, saying, "We cannot sacrifice our safety to the murderous greed of drug cartels." Brewer said she would order the state police training agency to form guidelines to train officers and protect against racial profiling.

Brewer spent as much time during her remarks talking about diversity and the need to avoid racial profiling as she did about fighting crime and protecting Arizona from illegal immigration. "People across America are watching Arizona, seeing how we implement this law, ready to jump on the slightest misstep," she said.

But the law's opponents were highly skeptical that it could be enforced without police singling out Latinos. One provision of the law prevents police from using race "solely" to form a suspicion about someone's legality, but the law does not prevent race from being a factor.

The bill, SB 1070, landed on Brewer's desk Monday afternoon; she had until Saturday to sign or veto it. The Republican governor, who is advocating a 1% sales tax hike on the ballot next month, faces a tough primary in August. Virtually every Republican in the state Legislature voted for the bill.

Hundreds of high school students left classes this week in protest, pouring into the plaza outside the state Capitol and urging a veto. Religious leaders and police chiefs - and thousands of callers to the governor's office - pressed for Brewer to reject the bill. Some Arizona officials argued it would stigmatize the state much as its past refusal to honor the birthday of the Rev. Martin Luther King Jr. U.S. Rep. Raul M. Grijalva, a Democrat who represents southern Arizona, called for a convention boycott of his own state.

But a recent poll showed that 70% of state voters supported the measure - even though 53% said it could lead to civil rights violations. Because of that broad support in a state that is the main gateway for illegal immigrants into the United States, people on both sides of the debate have long expected Brewer to sign the bill.

"It's a sad day for the country," said Alfredo Gutierrez, a former Arizona Senate majority leader who fought against the bill. "This is the most oppressive piece of legislation since the Japanese internment camp act" during World War II, he added.

Supporters of the measure were elated.

"Arizona is actually taking the lead in doing what the president is failing to do, which is to protect the interests of the people of Arizona," said Ira Mehlman of the Federation for American Immigration Reform. "What the Legislature and Gov. Brewer are saying is, 'If the president won't do it, we're going to do it ourselves.' "

Unless opponents can stop it with lawsuits, the law will take effect 90 days after the legislative session ends this month or in May.

The law creates the new misdemeanor and requires police to enforce it. The law's supporters argue that fears of widespread racial profiling are overblown and that the measure will instead be used sparingly by police to augment investigations into crimes.

But many of those supporters also cite the office of Maricopa County Sheriff Joe Arpaio, whose deputies have for years enforced federal immigration laws, as a model for how the rest of Arizona police should operate. Arpaio's office is regularly accused of racially profiling Latinos and is subject to a civil rights investigation by the U.S. Department of Justice.

"The state of Arizona has turned its back on everything we call American," said Father Glenn Jenks of the Arizona Interfaith Network, one of a wide range of religious groups that urged a veto. "The Hispanic community is already terrorized. Many of them are saying, 'We're going to get out of here' - and not just illegal immigrants."

In Washington, Obama cited the law during a citizenship ceremony for 24 active-duty service members as an example of why the nation needs a comprehensive immigration overhaul.

"Our failure to act responsibly at the federal level will only open the door to irresponsibility by others," Obama said in the Rose Garden ceremony. "That includes, for example, the recent efforts in Arizona, which threaten to undermine basic notions of fairness that we cherish as Americans, as well as the trust between police and our communities that is so crucial to keeping us safe."

Obama has devoted more energy to passing an immigration bill this year, but the prospects remain dim, with only one Republican senator voicing measured support and some Democrats hoping it gets put off.

In Arizona, officials said they acted because the federal government had failed to secure their border with Mexico, making the state vulnerable to drug traffickers and human smugglers who are blamed in the killing of a rancher on his land in southern Arizona last month.

Secretary of Homeland Security Janet Napolitano, Brewer's predecessor, said the law would hinder federal law enforcement efforts in the state.

"With the strong support of state and local law enforcement, I vetoed several similar pieces of legislation as governor of Arizona because they would have diverted critical law enforcement resources from the most serious threats to public safety and undermined the vital trust between local jurisdictions and the communities they serve," she said in a statement.

State Sen. Russell Pearce, who wrote the legislation, scoffed at Obama's opposition, contending that the president stood "against law enforcement, our citizens and the rule of law."

Gutierrez said Obama's statements heartened activists who were angered by recent federal immigration raids and the lack of an immigration bill in Washington. "People were beginning to feel mighty abandoned by the administration," he said, "and that helped calm it."