LOS ANGELES -- The Board of Supervisors violated the constitutional and voting rights of Los Angeles County's huge Hispanic population when it created district voting boundaries in 1981, a federal judge ruled yesterday.

"It is undeniable . . . that the Los Angeles County Board of Supervisors knew that by adopting the 1981 redistricting plan, they were further impairing the ability of Hispanics to gain representation on the board," U.S. District Judge David V. Kenyon wrote. Hispanics are estimated to make up 35 percent of the county's 8.7 million population.

Officials of the Mexican American Legal Defense and Educational Fund (Maldef) were jubilant at the ruling. "We are very pleased and very excited about this because this case is one of the biggest ones we've seen in California," spokeswoman Alicia Maldonado said.

Board of Supervisors officials were not immediately available for comment.

Maldef, the American Civil Liberties Union Foundation of Southern California and the Justice Department sued in 1988, accusing the board of drawing district boundaries in such a way as to keep a Hispanic representative from being elected.

Maldonado said it was not immediately clear how the violations would be resolved, but she said Kenyon is scheduled to discuss the ruling with all parties Thursday.