DALLAS - Latino activists and civil liberties advocates asked a federal judge Tuesday to block a voter-approved ordinance that prohibits landlords from renting apartments to most illegal immigrants in the Dallas suburb of Farmers Branch.

The Mexican American Legal Defense and Educational Fund and the American Civil Liberties Union, which have already sued the city, requested the temporary restraining order in U.S. District Court. A group of merchants also suing the city filed a separate request Tuesday to stop the ordinance's enforcement, activist Carlos Quintanilla said

The ordinance, scheduled to take effect next Tuesday, requires apartment managers to verify that renters are U.S. citizens or legal immigrants before leasing to them, with some exceptions. Violators face fines of up to $500.

"It is unfortunate that the residents of Farmers Branch have chosen to implement a law which is not only bad policy, but is likely also unconstitutional," said Lisa Graybill, legal director for the ACLU of Texas. "Now the issue will have to be resolved in federal court."

The city is preparing a response to the request, said Matthew Boyle, an attorney representing Farmers Branch. City Councilman Ben Robinson said he believes the groups are wasting their time.

City council members first approved the ban in November without discussion, then revised it in January to include exemptions for minors, seniors and some mixed-status families. On Saturday, residents voted to approve the ban with 68 percent of voters in favor, according to unofficial results.

Councilman Tim O'Hare, the ordinance's lead proponent, contends the city's economy and quality of life will improve if illegal immigrants are kept out. O'Hare declined to comment Tuesday.

Since 1970, Farmers Branch has changed from a small, predominantly white bedroom community with a declining population to a city of almost 28,000 people, about 37 percent of them Hispanic, according to the U.S. Census Bureau.

Farmers Branch already faces four lawsuits brought by civil rights groups, residents, property owners and businesses, who say the ordinance discriminates and puts landlords in the precarious position of acting as federal immigration officers. Their attorneys argue the ordinance attempts to regulate immigration, a duty that is exclusively the federal government's.

Nationwide, more than 90 local governments have proposed, passed or rejected laws prohibiting landlords from leasing to illegal immigrants, penalizing businesses that employ them or training police to enforce immigration laws.