Brewer was under intense pressure to not sign the legislation. Civil rights groups have decried the sweeping measure as opening the door to racial profiling and sowing distrust between Hispanics and the law enforcement groups charged with keeping them safe. Others said the law will pull resources from fighting more-serious crimes.
Thousands of people wrote or called the governor's office, with a 10-to-one majority opposing the bill, a spokeswoman said.
"I don't think anything has been this extreme until this point," said Bridgette Gomez, a 24-year-old math tutor. "The evil is racial profiling, to think that you're going to always have to show identification. Because I'm tan, I must be illegal."
But supporters of the law, including U.S. Sen. John McCain, R-Ariz., have said it will help solve an illegal immigration crisis the federal government so far has not acted swiftly enough to contain.
"Illegal is illegal," said the bill's sponsor, Republican state Sen. Russell Pearce. "We'll have less crime. We'll have lower taxes. We'll have safer neighborhoods. We'll have shorter lines in the emergency rooms. We'll have smaller classrooms."
An estimated 10.8 million immigrants live illegally in the U.S., according to the most recent Department of Homeland Security figures. About 460,000 live inside Arizona's borders.
Now that the Arizona bill has become law, it likely will face constitutional challenges.
President Obama said he's instructed the Justice Department to "closely monitor" the situation and "examine the civilian rights" and other implications of the legislation.
The Mexican American Legal Defense Fund (MALDEF) and other groups are also preparing to challenge the legislation.
"The Constitution is pretty clear about having one set of rules," said Thomas A. Saenz, general counsel and president of MALDEF. "Now, you have the state of Arizona coming along and creating an obstacle to federally mandated priorities."
Still, state Sen. Pearce, a former deputy in the Maricopa County Sherriff's Office, which is known for cracking down on illegal immigrants, said he's merely trying to enforce law that's already on the books.
"Illegal is not a race. It's a crime. And in Arizona, we're going to enforce the law ... without apologies," he said. "It's just that simple."
California attempted to pass a similar measure in 1994 -- Proposition 187 -- that was designed to keep illegal immigrants from using health, education and other social services.
Even though it passed, it was struck down by a federal court on the basis of constitutionality.
Similar legal challenges against Arizona are inevitable, Saenz said, and it will likely end up costing the state millions of dollars.
"Arizona is going to face very serious consequences if it enacts it," Saenz said, comparing it to the experience in California, where the legislation was a "tremendously wasteful diversion of resources."
"There was a palpable impact on international trade to California, in particular," Saenz said. "It became clear over time that Mexican companies began to take their commerce through Texas and other border states because of pervasive hostility."