PHOENIX (Reuters) - Arizona Governor Jan Brewer 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.

Immigrants hide from a border patrol vehicle while waiting a chance to cross into the United States at the border fence on the outskirts of the Tijuana, Mexico, September 19, 2009. REUTERS/Jorge Duenes

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” -- which critics charge will open the door to racial profiling.

Immigration is a bitterly fought 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 Latino 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.”

The bill -- passed by the Republican-controlled state Senate this week and signed by Brewer, a Republican -- is expected to spark a legal challenge and already is 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,” Obama said earlier on Friday at a ceremony swearing in new U.S. citizens.

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.

Brewer said that Arizona acted because Washington had not, and she said that 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.

BACKLASH FEARED

While Republican-backed measures such as this anger immigration 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.

The Arizona law is the harshest of a growing number of immigration measures passed by U.S. states.

It requires state and local police officers to arrest those unable to provide documentation proving they are in the country legally. It also makes it a crime to transport someone who is an illegal immigrant, and to hire day laborers off the street.

The passage of the law, which is slated to take effect 90 days after the current legislative session adjourns, prompted immediate and strong reactions.

“It’s a great day for the people of Arizona because once implemented we will all be safer,” state Representative John Kavanagh, a Republican who backed the measure, told Reuters.

“The message this sends to the country is that when the federal government lets us down we know what to do and the message it sends to illegal aliens is don’t come to Arizona,” he added.

Janet Murguia, president of civil rights group the National Council of La Raza and an opponent of the measure, expressed deep disappointment “that Governor Brewer chose politics over sound policy,” in signing the bill into law.

“The passage of (the bill) will legitimize racial profiling in Arizona and it goes against our laws and our values as a country,” she added.