By David Schwartz and Tim Gaynor

PHOENIX (Reuters) - Arizona police on Wednesday began enforcing a controversial "show-your-papers" provision of a state law targeting illegal immigration as civil rights groups prepared to document allegations of racial profiling.

Police in the border state with Mexico are now authorized to begin conducting immigration status checks of anyone they stop for any reason and suspect of being in the country illegally after a federal judge on Tuesday lifted an injunction against the provision requiring such checks.

The measure, upheld by the U.S. Supreme Court in June, is part of a broad Arizona clampdown on illegal immigration signed into law in 2010 by Republican Governor Jan Brewer, an outspoken foe of President Barack Obama's administration on immigration.

Brewer has said the law was needed because of the federal government's failure to secure the border with Mexico. She said enforcement would be free of any racial profiling.

"It's definitely a new phase, and one where we'll be looking very carefully to monitor for civil rights violations in the state," said Karen Tumlin, managing attorney with the National Immigration Law Center, one of a coalition of groups that challenged the law.

"There is a hotline set up ... where folks can report any violations or questionings or detentions that happen under the law," she added.

The U.S. Supreme Court, in upholding the measure even as it struck down three other Arizona immigration provisions, has left open the door for legal challenges, saying constitutional or other challenges could proceed once the measure took effect.

Rights activists who have fought a two-year legal battle against the measure have said they are ready to go to court quickly if they learn of instances of racial profiling or illegal detention.

Opponents of the measure are also pinning hopes on a legal challenge filed with the 9th U.S. Circuit Court of Appeals that seeks an injunction to halt the law's enforcement.

Police in Phoenix and Tucson said they were now enforcing the law, although law enforcement agencies from across the state have said they expect little change in their policing. Police did not specify what enforcement steps officers were taking on patrol.

HOTLINE

Even before the injunction on the measure was lifted, the American Civil Liberties Union said a bilingual hotline on the law had already taken 3,500 calls in Arizona, where nearly a third of the population of 6.5 million is Hispanic.

About 50 activists rallied outside the federal Immigration and Customs Enforcement, or ICE, agency offices in central Phoenix later on Wednesday, in a gesture of defiance toward the law.

"The big message is we are not afraid, that undocumented people are here standing outside of ICE and telling everyone that ... we are organized and that we are not going away," said Carlos Garcia, of grass-roots group Puente Arizona. Puente is Spanish for bridge.

In a statement after Tuesday's court ruling, Brewer said police " bring their training and experience to this important task, as well as a solemn commitment to serving the public, protecting our citizens and upholding the law.

"That means all of our laws, including those barring racial profiling or discrimination," she added.

The Obama administration battled the measure on the grounds that it interfered with federal immigration powers, and Arizona was expected to get limited help in enforcing the provision from the federal government.

Amber Cargile, the spokeswoman in Phoenix for ICE, said U.S. Department of Homeland Security officials in Arizona had been told not to respond to police requests relating to immigration enforcement unless it met the government's priorities.

Those include convicted criminals, individuals previously removed from the United States and recent border crossers, she said in a statement.

"DHS will continue to telephonically comply with its legal requirement to verify an individual's immigration status upon request," Cargile said in a statement.

Last year, ICE received about three calls a day to its Phoenix unit seeking verification. Cargile said the agency was receiving a "normal volume" of calls on Wednesday.

(Writing by Tim Gaynor; Editing by Cynthia Johnston and Peter Cooney)