The Republican Governor of the American State of Arizona on Friday signed into effect the toughest immigration law in the United States, which President Barack Obama singled out as a "misguided" effort that showed the need for national reform. With hundreds of protesters in Phoenix shouting that the bill would lead to civil rights abuses, Governor Jan Brewer said critics were "overreacting" and that she wouldn't tolerate racial profiling. "We in Arizona have been more than patient waiting for Washington to act," Brewer said after signing the law. "But decades of inaction and misguided policy have created a dangerous and unacceptable situation." Police in the border state with Mexico will now be required to determine if people are in the country illegally if there is "reasonable suspicion" that is the case -- which critics charge will open the door to racial profiling. Immigration is a bitterly contested issue in the United States, where some 10.8 million illegal immigrants live and work in the shadows. But until recently it has been eclipsed at the national level by other issues such as healthcare and financial reform, angering many Latin American supporters of Obama. Several thousand people gathered outside the state capitol in Phoenix ahead of the governor's announcement, mostly opponents carrying signs saying "We Are Human" and "Enough is Enough." Mexico's foreign minister, Patricia Espinosa, said the bill could damage the relationship between Mexico and Arizona, which have strong commercial ties. An estimated 460,000 illegal immigrants live in the desert state, which also straddles the main point of entry for illegal immigrants crossing into the United States from Mexico. The bill -- passed by the Republican-controlled state Senate this week and signed by Ms. Brewer -- is expected to spark a legal challenge and is already a hot issue in the run-up to U.S. congressional elections in November. "If we continue to fail to act at a federal level, we will continue to see misguided efforts opening up around the country," President Obama said earlier on Friday at a ceremony swearing in new U.S. citizens. Concern over border-related crime spiked late last month in Arizona -- where Senator John McCain faces a tough challenge in the Republican primary race from fiery conservative J.D. Hayworth -- after a prominent rancher was shot dead by a suspected smuggler close to the Mexican border. Ms. Brewer said the new law strengthened Arizona and would help keep it safe from drug cartels and other threats. "It protects all of us, every Arizona citizen and everyone here in our state lawfully and it does so while ensuring that the constitutional rights of all in Arizona remain solid," she said. She said Arizona had acted because Washington had not, and she said police would be trained on the concept of what constitutes "reasonable suspicion" that someone is an illegal immigrant. "I do not know what an illegal immigrant looks like," she said in response to a question. While Republican-backed measures like this anger reform supporters, the Democratic Party fears a backlash from Latinos in the November congressional elections because of a lack of movement on the issue since Obama took office.