Activists are planning a challenge of their own, hoping to block the law from taking effect by arguing that it encroaches on the federal government's authority to regulate immigration and violates people's constitutional rights by giving police too much power.
The measure - set to take effect in late July or early August - would make it a crime under state law to be in the U.S. illegally. It directs state and local police to question people about their immigration status if there is reason to suspect they are illegal.
"If you look or sound foreign, you are going to be subjected to never-ending requests for police to confirm your identity and to confirm your citizenship," said Alessandra Soler Meetze, executive director of the American Civil Liberties Union of Arizona, which is exploring legal action.
Employees at the Capitol came to work Monday to find that vandals had smeared swastikas on the windows. And protesters gathered for a second straight day to speak out against a law they say will lead to rampant racial profiling of anyone who looks Hispanic.
The White House would not rule out the possibility that the administration would take legal action against Arizona. President Barack Obama, who warned last week that the measure could lead to police abuses, asked the Justice Department to complete a review of the law's implications before deciding how to proceed.
Mexican President Felipe Calderon said the law is discriminatory and warned that trade and political ties with Arizona will be seriously strained by the crackdown.
Currently, many U.S. police departments do not ask about people's immigration status unless they have run afoul of the law in some other way. Many departments say stopping and questioning people will only discourage immigrants from cooperating to solve crimes.
Under the new Arizona law, immigrants unable to produce documents showing they are allowed to be in the U.S. could be arrested, jailed for up to six months and fined $2,500. That is a significant escalation of the typical federal punishment for being here illegally - deportation.
People arrested by Arizona police would be turned over to federal immigration officers. Opponents said the federal government could thwart the law by refusing to accept them.
Supporters of the law said it is necessary to protect Arizonans from crimes committed by illegal immigrants. Arizona is home to an estimated 460,000 illegal immigrants and is the nation's busiest gateway for people slipping into the country.
Republican Gov. Jan Brewer, who signed the bill on Friday, said Arizona must act because Washington has failed to stop the flow of illegal immigrants and drugs from Mexico. Brewer has ordered state officials to develop a training course for officers to learn what constitutes reasonable suspicion that someone is in the U.S. illegally.
The crux of opponents' arguments is that only the federal government has the authority to regulate immigration.
"If every state had its own laws, we wouldn't be one country; we'd be 50 different countries," said Thomas Saenz, president and general counsel of the Mexican American Legal Defense and Educational Fund.
Kevin Johnson, dean of the law school at the University of California-Davis and an immigration law professor, said such a lawsuit would have a very good chance of success. He said the state law gets into legal trouble by giving local law enforcement officers the authority to enforce immigration laws.
"States can't give them that power," Johnson said. "The federal government could if it wanted to, but it hasn't."
However, Gerald Neuman, a Harvard Law School professor, said Arizona could make a compelling legal argument that it has overlapping authority to protect its residents.
Johnson said opponents could also argue that the law could violate their Fourth Amendment protection against unreasonable search and seizure because it gives police officers broad authority to determine who should be questioned.
Kris Kobach, a University of Missouri-Kansas City law professor who helped write the Arizona legislation, said he anticipated legal challenges and carefully drafted the language. He said the state law is only prohibiting conduct already illegal under federal law.
In a statement Friday, Homeland Security Secretary Janet Napolitano said the state's new law would probably hinder law enforcement in dealing with more serious crimes. Napolitano vetoed similar proposals when she was Arizona governor.
"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.
San Francisco City Attorney Dennis Herrera urged policymakers in the city to stop dealing with Arizona and Arizona businesses. Leaders in Mexico and California also demanded a boycott, as did civil rights leader Al Sharpton.
The law has strong public support in Arizona, where passions have been running high since a rancher was killed close to the Mexican border last month, apparently by drug smugglers from across the border.
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Associated Press Writers Eileen Sullivan and Darlene Superville in Washington, Julianna Barbassa in San Francisco and Paul Davenport in Phoenix contributed to this report.
