FARMERS BRANCH, Texas—Activists filed a lawsuit Monday against the Dallas suburb that recently banned landlords from renting apartments to most illegal immigrants, claiming minorities are underrepresented because of the at-large city council system.

The federal lawsuit was filed in Dallas on behalf of three Latino voters who live in Farmers Branch, where voters recently approved the apartment rental ordinance.

The lawsuit seeks the creation of single-member districts, in which a city council member is elected to represent a specific section of the city. Both large and small cities with diverse racial makeup use the system, said Rolando Rios, the attorney leading the suit.

Activists say if the method had been in place, at least one Latino candidate would have been elected to the council and could represent the group.

Since 1970, Farmers Branch has changed from a small, predominantly white 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.

The city had not yet been served with the lawsuit, said Farmers Branch spokesman Tom Bryson.

A federal judge was also expected to decide Monday whether Farmers Branch can enforce the apartment rental ordinance.