Former top FBI officials W. Mark Felt and Edward S. Miller were fined a total of $8,500 in U.S. District Court here yesterday on their convictions of conspiracy for authorizing illegal searches at homes of friends and relatives of the radical Weather Underground in the early 1970s.

Chief Judge William Bryant's terse pronouncement of the penalties -- $5,000 for Felt and $3,500 for Miller -- closed a sensitive and controversial chapter in the Justice Department's criminal prosecution of the two veteran FBI men who said they believed they had the power to approve the warrantless entries for the sake of national security.

Felt, 67, began his FBI career as a street agent and ended it 31 years later in 1973 as the bureau's No. 2 man. Miller, 57, was former chief of the domestic intelligence division and, like Felt, had risen to his position through the bureau ranks. Each had faced a sentence of up to 10 years in jail and a $10,000 fine or both on the single conspiracy conviction.

Felt, summoned by Bryant to hear his sentence first, turned from the courtroom lectern after the fine was imposed and seemed barely able to conceal a smile of relief. There was a prolonged stir of voices in the courtroom, which was packed with retired FBI agents, when it became clear that Bryant did not intend to send either man to prison for their conviction. Neither Bryant nor the government made any comment about the sentences.

"I'm glad the sentence was no stiffer than it was, that's a relief," Felt said outside the courthouse. Earlier, Miller had said, "I think I'm a little in shock . . . it could have been 10 years and $10,000 worse."

After an eight-week trial, Felt and Miller were convicted by a jury in Bryant's courtroom Nov. 6 of authorizing illegal break-ins -- known in the bureau as "black bag jobs" -- at the homes of friends and relatives of fugitive Weathermen, in violation of their civil rights.

Both Felt and Miller told Bryant they had nothing to say before sentencing. Instead, Felt's principal lawyer, Brian P. Gettings, and Miller's lead attorney, Thomas A. Kennelly, addressed Bryant, stressing what they said was the personal anguish their clients had undergone and urging the judge not to send them to jail. Gettings described the trial as lengthy, expensive, "and to be sure, an important case."

"Part of the purpose of the prosecution was to vindicate a principle . . . certainly the conviction vindicated the principle at great personal cost to mr. Felt," Gettings said. Miller's lawyer, Kennelly told Bryant that his client was "doing his job as he saw it . . . and for the sole motive of protecting the people of the United States from the actions of radical terrorists."

After sentencing was done, Felt and Miller waded through a small crowd of supporters, mostly retired Bureau men, who pushed into the aisle of the courtroom to shake hands and offer congratulations.

The Society of Former Special Agents of the FBI Inc. had raised and spent more than $800,000 for Felt's and Miller's defense.Additional money collected by that group financed lawyers for more than 130 agents who were linked to the Justice Department's long investigation into the black bag cases that ended with the indictment of Felt, Miller and L. Patrick Gray III, acting bureau chief at the time of the break-ins.

Felt and Miller acknowledged they approved the entries but said they were justified for national security reasons as part of a hunt for the Weathermen fugitives in an effort to stop bombings and terrorism for which the radical group had claimed credit. Both men said they had general authority to conduct the entries from Gray.

Gray was indicted in April 1978 with Felt and Miller on the same conspiracy charge. Last Thursday, however, the government dropped its indictment of Gray, admitting it could not prove Gray had a role in authorizing the surreptitious entires.

Felt and Miller declined comment yesterday when asked by reporters if they would seek a presidential pardon from Ronald Reagan, but both men said they would move ahead with plans to take their case to the U.S. Court of Appeals. Their fines need not be paid until they have exhausted their appeals.

". . . Remember, I'm now a convicted felon," Felt said yesterday. And that, he said, is a "very serious blemish on my whole life."