PHOENIX — Letters purporting to offer information about illegal immigrants are included in the evidence to be introduced in the class-action civil-rights trial against Sheriff Joe Arpaio and his Maricopa County Sheriff's Office that began Thursday in U.S. District Court in Phoenix.

The elected sheriff and his office are accused of engaging in a pattern of discriminatory policing during large-scale operations known as suppression patrols, unfairly singling out Latinos — including citizens and legal immigrants — for stops, questioning and detention.

In one letter to the sheriff, a writer described dark skin as "the look of Mexican illegals," urging the sheriff to go to a particular street corner on the northern edge of the city and "round them all up." Another complained about people speaking Spanish at a fast-food restaurant in Sun City, northwest of Phoenix. Yet another grumbled about day laborers gathered at a spot in nearby Mesa, asking when officers would check to see whether they were there "under legitimate circumstances."

Arpaio has repeatedly and vehemently denied the accusations, and legal experts have said that discriminatory intent is hard to prove.

In court Thursday, Tim Casey, a lawyer for the defendants, said: "There are two sides to every story. If the truth were anything like what the plaintiffs are suggesting, it would be a very disturbing picture."

Reality, Casey said, is much different. Race and ethnicity, he went on, have "nothing to do with it."

The American Civil Liberties Union and the Mexican-American Legal Defense and Education Fund are representing the plaintiffs.

This trial, and a broader lawsuit filed against Arpaio by the Justice Department in May, both focus on potential violations of civil rights. The six plaintiffs named in this litigation, filed nearly five years ago, represent Latinos who were stopped by Maricopa County sheriff's deputies since 2007.

Their lawyers will argue that the letters sent to Arpaio led to unlawful enforcement actions.

Maricopa County is also named in the litigation; its supervisors failed to stop Arpaio, the lawsuit asserts.

The bench trial, presided over by Judge G. Murray Snow, is scheduled to end Aug. 2. Arpaio, who is 80, will most likely testify next week.