The American Civil Liberties Union filed suit yesterday in U.S. District Court in Alexandria alleging that local election laws in Warrenton, Va., have been "maintained purposefully to dilute . . . and cancel out the voting strength of blacks."

The suit, filed on behalf of several black voters in the Fauquier County town, is the eighth one the ACLU has filed in Virginia towns in its attempt to ensure that black voters are fairly represented.

Warrenton has a population of 3,907 and 21 percent of its residents are black, according to census statistics. No black person has ever been elected to office there.

The ACLU wants the Town Council to switch from an at-large electoral system -- in which any Warrenton voter gets to choose seven council members -- to a ward system that is based on locality within the city.

"We negotiated with them for months, but they wouldn't be moved," said Chan Kendrick, executive director of the Virginia ACLU. "They are violating the federal voting rights act."

The suit alleges that the town is using voting "procedures which enhance the opportunity for discrimination against blacks." The effect of the at-large policy, the suit alleges, is to deny equal political access to blacks.

The ACLU has proposed a plan that divides Warrenton into five wards with 65 percent of the town's black residents in one district. The district would contain the two largest black communities in Warrenton.

"We don't accept what the ACLU has in mind for Warrenton," said J. Willard Lineweaver, the town's mayor. "But as long as this is in litigation I will have no other comment to make."

Because at-large elections make no distinction for the geographical residence of the candidates, it is hard for minorities to make their voice heard in local elections. In 1982, Congress enacted extensive amendments to the Voting Rights Act of 1965 that state that voting procedures are illegal if they have a racially discriminatory result.

Since that time, the Virginia ACLU has won several voting rights suits, including a victory that forced the state to abolish at-large districts for statewide offices. Farmville, Prince Edward County, Halifax and Hopewell have had to institute a ward system for local elections as a result of ACLU suits.

"This is a typical example of discrimination in an area where there had been a history of it," said Vic Glasberg, an ACLU lawyer. "We just want to see people get a chance to make their votes count. They are entitled to that."