By Tim Gaynor and David Schwartz

PHOENIX, Sept 25 (Reuters) - A U.S. federal appeals court on Tuesday rejected a bid by a coalition of civil and immigrants rights activists to prevent police from enforcing an Arizona provision that is at the heart of the fierce national debate over illegal immigration.

The 9th U.S. Circuit Court of Appeals in San Francisco denied an emergency motion for an injunction blocking the "show your papers" provision of SB 1070, the state's crackdown on illegal immigrants, pending appeal.

The provision requires police to verify the citizenship or immigration status of people arrested, stopped or detained if there is a reasonable suspicion that they are in the country unlawfully.

It went into effect on Sept. 18 after a U.S. district judge lifted an injunction blocking it.

In June the U.S. Supreme Court confirmed that three other key provisions of SB 1070 were unconstitutional, but declined to block the "show me your papers" provision. Several other parts of SB 1070 are blocked by separate injunctions issued by the district court.

Arizona Republican Governor Jan Brewer signed the state crackdown on illegal immigrants into law in April 2010, saying that the federal government had failed to secure the state's border with Mexico.

Brewer is an outspoken foe of Democratic President Barack Obama's administration on immigration.

In a statement Tuesday, the governor said she was under "no illusion that opponents of SB 1070 will stop their baseless allegations and call off their teams of lawyers."

"Know this: They will not succeed. The State of Arizona stands firmly in support of the rule of law, in defense of our citizens and together with our brave men and women in uniform," she added.

Karen Tumlin, managing attorney with the National Immigration Law Center which was among a coalition that challenged the law, said the group is exploring its legal options.

"We need to continue the fight because of the unconstitutional harm it will unleash in Arizona" Tumlin said. "First and foremost, we are concerned about unlawful detention and individuals who may be profiled based on their manner of speech or the color of their skin."

Obama challenged Arizona's law in court two years ago, saying the U.S. Constitution gives the federal government sole authority over immigration policy.

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