A judge in Arizona ruled Tuesday that police can immediately start enforcing the most contentious section of the state's immigration law, marking the first time officers can carry out the so-called "show me your papers" provision.

The decision by U.S. District Judge Susan Bolton is the latest milestone in a two-year legal battle over the requirement. It culminated in a U.S. Supreme Court decision in June that upheld the provision on the grounds that it doesn't conflict with federal law.

Now, with the requirement finally in full effect, both sides are anxious to see the outcome.

The supporters want local police to use it vigorously, but worry federal immigration officials won't respond to calls to come arrest people.

"I am mulling what I will do if they don't respond," said Maricopa County Sheriff Joe Arpaio, who more than any other police boss in the state pushed the bounds of immigration enforcement. "I don't feel comfortable letting the illegal alien back on the street."

Federal officials said they will check people's immigration status when officers call. But they'll only send an agent to arrest someone if it fits with their priorities, such as catching repeat violators and those who are a threat to public safety and national security.

Meanwhile, civil rights advocates are preparing for a battle.

— They're stepping up efforts to staff a hotline that fields questions about what people's rights are in case officers question their immigration status.

— If a police agency plans a special immigration patrol, volunteers armed with video cameras will be sent to capture footage, said Lydia Guzman, leader of the civil rights group Respect-Respeto.

— The law's opponents are spreading out across the state, asking police departments not to enforce the provision. Doing so could open officers up to lawsuits from people who could claim the agencies aren't fully enforcing the law.

The incentive to not enforcing the law, said Carlos Garcia, an organizer for the Puente Movement: better cooperation of immigrants who would be more likely to report crimes.

Arizona's law was passed in 2010 amid voter frustration with the state's role as the busiest illegal entry point into the country. Five states — Alabama, Georgia, Indiana, South Carolina and Utah — have adopted variations on Arizona's law.

This section of the law requires that officers, while enforcing other laws, question the immigration status of those suspected of being in the country illegally. The "show me your papers" name comes from opponents.

It's a tool for local police, said Republican Gov. Jan Brewer, who signed the measure, but won't cure the state's immigration woes.

"Only the federal government has the resources and responsibility necessary to achieve that," Brewer said.

The law's journey to this point has taken many twists and turns. Bolton is the judge who initially blocked it after the Obama administration challenged it on the grounds that federal immigration law trumps state law.

The case made it all the way to the U.S. Supreme Court. There, justices barred police from enforcing other parts of the law, including a requirement that immigrants obtain or carry immigration registration papers. But they allowed the questioning requirement — to supporters the most important part — to move forward.

The latest challenge from a coalition of civil rights, religious and business groups — which Bolton denied earlier this month— said Latinos in Arizona would face systematic racial profiling.

But Bolton agreed with the state's lawyers that the law's opponents were merely speculating on those claims. She did leave the door open to challenges once the law is in effect, if the claims can be proven.