The Supreme Court yesterday gave local jurisdictions greater freedom to refashion their electoral systems and forms of government without interference from the federal Voting Rights Act.

The justices said the act, while barring voting-law modifications that increase discrimination, does not obligate jurisdictions to reduce existing levels of bias.

The 6-to-3 decision, a defeat for Mexican Americans in Texas, brought a strong dissent from Justice Thurgood Marshall, who said it allowed cities and counties to "perpetuate" discrimination against minority voters. It allows "adoption of a discriminatory election scheme, so long as the scheme is not more discriminatory than its predecessor," Marshall wrote.

But civil rights lawyers said the ruling's impact was unclear because in yesterday's case the court did not consider amendments to the act that were approved in 1982. In addition, they said, changes with a discriminatory intent are still barred whether or not they worsen the effect of the existing system.

The ruling applies to provisions in the act that require the Justice Department or a federal judge to "pre-clear" modifications of local government in states with a history of voting discrimination. Changes may be disapproved if either their purpose or their effect is discriminatory. Only the "effect" issue was before the court yesterday in City of Lockhart vs. United States.

Before 1973, Lockhart, 30 miles south of Austin, had a mayor and two commissioners elected every two years on an at-large basis. Each commissioner's post was filled in a separate contest--by "numbered posts"--rather than in a single race with victory going to the two highest vote-getters.

Mexican Americans, who constituted 47 percent of Lockhart's population, say such a system discriminates against minorities by discouraging the concentration of strength behind a single candidate. No Mexican American was ever elected in Lockhart.

Lockhart changed its system when it was given home rule by the Texas Legislature. It established a government consisting of a mayor and four council members. But it retained the at-large voting system with the numbered posts and staggered terms. Though a Mexican American was elected under the new system, others challenged it as nonetheless discriminatory.

Justice Lewis F. Powell Jr. rejected the city's contention that pre-clearance was unnecessary. But he said that, while "there may have been no improvement" in the voting strength of the Mexican Americans, "there has been no retrogression either." Citing a 1976 Supreme Court ruling (Beer vs. U.S.), Powell said only retrogression would trigger disapproval of the changes.

The Mexican Americans still can attempt to prove that the change was intentionally discriminatory. They can also challenge the system under another provision of the Voting Rights Act.

Justice Harry A. Blackmun supported Marshall's objections. Justice Byron R. White dissented from much of the opinion as well, saying that "at the very least" he would allow the lower court to determine how the 1982 amendments affected the case.

Also yesterday:

* The court ruled, 5 to 4, that state governments are not exempt from federal antitrust laws when they compete with private business by purchasing products for resale. The case, Jefferson County Pharmaceutical Association Inc. vs. Abbott Laboratories et al, involves government-operated pharmacies at public hospitals that paid lower prices for drugs than did private pharmacists.

* The court said federal laws that bar convicted felons from the firearms business apply even to people whose criminal records have been expunged in a state court. Blackmun, writing the 5-to-4 opinion in Dickerson vs. New Banner Institute Inc., said a conviction remains a conviction, regardless of expungement.

* Because of a 4-to-4 tie, the court failed to resolve an important question: whether improper jury instructions in a criminal trial must always result in reversal of a conviction. Justice John Paul Stevens declined to agree with the reasoning of either side in Connecticut vs. Johnson.