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The office of an Arizona lawman who styles himself America's toughest sheriff improperly targeted Hispanics in routine patrols for undocumented immigrants, a federal judge has found.

Maricopa County Sheriff Joe Arpaio was sued by a group of Hispanic drivers who said his deputies relied on race when choosing whom to stop.

Judge Murray Snow ruled his deputies may not factor in Hispanic ancestry when making law-enforcement decisions.

Mr Arpaio denied the allegations.

The sheriff's office "has never used race and will never use race in its law-enforcement decisions", said his lawyer Tim Casey, who promised to appeal the decision.

"This is an important victory that will resound far beyond Maricopa County," said Cecillia Wang, director of the American Civil Liberties Union Immigrants' Rights Project.

"Singling people out for traffic stops and detentions simply because they're Latino is illegal and just plain un-American."

The ruling was handed down on Friday, eight months after a seven-day trial.

At the trial, Maricopa County officials testified that the sheriff's anti-illegal immigrant squad targeted locations where Hispanic day labourers gathered to await work.

The judge found the deputies did not routinely check the immigration status of every day labourer, merely those who were of Hispanic origin. Mr Arpaio himself testified he would not investigate the immigration status of white people because he assumed they were in the US legally.

"The evidence demonstrates that the [Maricopa County Sheriff's Office] specifically equated being a Hispanic or Mexican (as opposed to Caucasian or African-American) day labourer with being an unauthorized alien," Judge Snow wrote in a .

Judge Snow also found the deputies relied on pretexts to pull over vehicles in which they suspected the passengers to be illegal immigrants.

The ruling bars Maricopa County deputies from using Hispanic ancestry or race in making law-enforcement decisions, including in decisions to stop a person and verify his or her authorisation to be in the US.

It also bars the deputies from detaining Hispanic vehicle passengers based on "reasonable belief, without more", that they are in the country illegally.

The case was brought by a Mexican tourist stopped outside a church where day labourers were known to congregate and subsequently detained for nine hours, the Arizona Republic reported.

He and others who sued Mr Arpaio alleged he ordered some immigration patrols based on citizens' complaints about brown-skinned people congregating or speaking Spanish.

Mr Arpaio's office has also been sued by the US justice department, which accused his office of multiple civil liberties violations, including punishing Hispanic jail inmates for speaking Spanish.

Mr Arpaio shot to prominence by forcing prisoners to wear pink underwear. In 2012, he suggested President Barack Obama's birth certificate was a forgery after an investigation by his "cold case posse".