PHOENIX (Reuters) - Arizona lawman Joe Arpaio, who calls himself America's toughest sheriff, was accused of racial profiling as a trial began on Thursday in a class-action lawsuit alleging he discriminated against Latinos in his crackdown on illegal immigration.

The case will test whether the Maricopa County Sheriff's Office can target those in the country illegally in immigration sweeps without racially profiling Hispanic citizens and legal residents.

The 80-year-old Arpaio, who is seeking re-election to a sixth term in November, has been a lightning rod for controversy over his aggressive enforcement of tough immigration laws in the border state with Mexico and his volunteer posse's investigation into the validity of President Barack Obama's birth certificate.

Arpaio was not in court on Thursday when lawyers for both sides in the class-action suit made opening statements before Judge Murray Snow in the non-jury bench trial at U.S. District Court in Phoenix.

"A fundamental value of our nation is equal protection under laws, regardless of race or ethnicity," plaintiff attorney Stanley Young said.

He added he would argue that Arpaio's office had "engaged in racial discrimination," and particularly cited his immigration sweeps, or "saturation patrols."

Tim Casey, a lawyer for Arpaio and the sheriff's office, said his side would prove there was no such profiling as alleged by the five plaintiffs in the lawsuit nor was there discrimination by the office in three stops cited in the suit.

"We have five plaintiffs on three stops. The evidence will show race and ethnicity had nothing to do with their traffic stops," he said.

The trial focuses attention again on Arizona, which claimed headlines last month when the U.S. Supreme Court upheld a key element of the state's crackdown on illegal immigrants requiring police to investigate those they stop and suspect of being in the country illegally.

Supporters said the law was needed because the U.S. federal government had failed to secure the porous Southwest border with Mexico. Obama's administration challenged it in court, saying the U.S. Constitution gives the federal government sole authority over immigration policy.

Arpaio faces a separate, broader lawsuit lodged by the U.S. Justice Department in May, alleging systematic profiling, sloppy and indifferent police work and a disregard for minority rights by him and county officials.

Protesters from both sides of the debate gathered outside the court from early morning toting flags and placards. One read, "Sheriff Joe does immigration sweeps, makes drug busts, what more do you want?"

"No to hate, no to fear" and "God forgive Arpaio," read others.

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Lawyers for the plaintiffs called Ralph Taylor, a criminal justice expert at Temple University in Philadelphia, who analyzed racial and ethnic patterns from traffic stops carried out by the office from 2007 to 2009, including about a dozen "saturation patrols" in the Phoenix valley.

Taylor told the court that when an officer ran a name check on the day of a major sweep, there was "a much higher likelihood" the name run would be Hispanic than non-Hispanic. He also testified that when an officer ran a check of at least one Hispanic name, the stop would take about two minutes longer.

During cross-examination, the defense called Taylor's study biased, arguing it failed to take into account variables including language difficulties that might lengthen stops involving Hispanics, and did not draw on complete data sets.

David Vasquez, 47, testified he was stopped by sheriff's officers in June 2008 during a sweep, ostensibly because he had a cracked windshield. The officer asked Vasquez if he spoke English and checked his driver's license and other documents, but did not cite him.

"I just found it funny that he asked me if I spoke English ... because I felt that I was being singled out," Vasquez said in testifying for the plaintiffs, adding he later told his wife he believed he was "pulled over for ‘driving while brown.'"

The five plaintiffs in the suit are Hispanics stopped by deputies and the Somos America immigrants' rights coalition. It was later expanded to include all Latino drivers stopped by the office since 2007.

The lawsuit was brought in the name of Mexican tourist Manuel Ortega Melendres, who was a passenger in a vehicle pulled over by deputies in 2007, ostensibly for speeding. Melendres was arrested, despite having a valid visa and producing identification, while the vehicle's white driver was neither cited nor taken into custody.

The defense counsel asked the deputy who stopped the vehicle, Louis DiPietro, if he relied on skin color in a decision to detain anyone, to which he replied, "No."