The Supreme Court, in a significant supplement to the newly extended Voting Rights Act, ruled yesterday that federal judges may order massive restructuring of racially biased electoral systems even when they don't find direct proof of intentional voting discrimination.

The 6-to-3 ruling upheld a lower court order forcing Burke County, Ga., to eliminate its at-large system of electing county commissioners. No black has ever been elected under the system although a majority of the county is black.

While the ruling does not go beyond the new amendments to the voting rights law signed Tuesday by President Reagan, civil rights lawyers said it could serve as an important tool in convincing lower court judges in cases all over the South to interpret the amendments liberally on behalf of minorities.

Dissenters said the decision constituted a retreat from the court's hard-line position, adopted in a 1980 Mobile, Ala., case, on the type of proof required to justify court-ordered electoral revisions.

According to the 1980 census, Burke County, population 20,000, is 53.6 percent black, with blacks constituting 38 percent of the registered voters. Blacks charged, as they have in similar communities across the South, that the at-large system was in part responsible for no blacks being elected.

Where voting is along racial lines, at-large systems tend to allow the registered majority to control the entire government, they say. District systems, the alternative to the countywide at-large system, at least allow black sections to elect one or two representatives.

The opinion noted that the lower courts also found that Burke County had a history of massive and overt discrimination in employment, schooling and allocation of resources ranging from good roads to plumbing.

Claiming violations of the Constitution's anti-discrimination provisions--the 13th, 14th and 15th Amendments--as well as of the Voting Rights Act of 1965, eight blacks sued in 1976, seeking to replace the at-large system with a district system.

In 1978, a U.S. District Court, later upheld by the 5th U.S. Circuit Court of Appeals, agreed, saying that while the system appeared neutral, evidence that it was purposeful was demonstrated by a "totality of circumstances" in Burke County including the other forms of blatant discrimination against blacks.

In 1980, the Supreme Court struck down a similar sweeping order imposed in Mobile because it was issued without a finding of intentional discrimination, as is required in other civil rights cases such as school desegregation.

The ruling was considered a major defeat for minorities, since most of these systems were created at the turn of the century, and finding actual "proof," or a "smoking gun," is considered impossible. They said it was enough to show that a system had a discriminatory "effect" in application.

Congress, despite vigorous opposition by the Reagan administration, rewrote the law to eliminate the "intent" requirement from the Voting Rights Act.

Yesterday's case was a chance for the court to explain how intent is to be proven, that is, whether a "smoking gun" is required. Agreeing with the lower courts in the Burke County case, Justice Byron R. White adopted a liberal test.

"Discriminatory intent need not be proven by direct evidence," White wrote. Then quoting from a previous ruling on discrimination in zoning, he said, " 'An invidious discriminatory purpose may often be inferred from the totality of the relevant facts, including the fact, if it is true, that the law bears more heavily on one race than another.' "

White said the failure of blacks to win elections is "important evidence of purposeful exclusion," though not sufficient in itself to warrant imposition of a remedy.

But he said lower courts had also found evidence of widespread bias in the past in all facets of Burke County government: in education ("as recently as 1969,"); in participation in political party activities; in requirements for holding certain public offices; in selecting grand jurors; in housing conditions ("Seventy-three percent of houses occupied by blacks lacked all or some plumbing facilities while only 16 percent of whites" had that problem), and in the unresponsiveness and insensitivity of white officials to black needs generally. "Evidence of historical discrimination," White wrote, "is relevant to drawing an inference of purposeful discrimination, particularly in cases such as this one where the evidence shows that discriminatory practices were commonly utilized, that they were abandoned when enjoined by courts or made illegal by civil rights legislation, and that they were replaced by laws and practices which, though neutral on their face, serve to maintain the status quo."

Justice Lewis F. Powell Jr., joined by Justice William H. Rehnquist, wrote a dissent saying that the decision left lower court judges too much discretion, "free from any standard propounded by this court." They said this was exactly the kind of situation the court found unacceptable in the Mobile case. " . . . . The court's opinion cannot be reconciled persuasively with that case . . . . "

Justice John Paul Stevens also dissented.