President Carter attmepted to seize the initiative in the Billy Carter affair yesterday by announcing that he is eager to testify in person before a Senate subcommittee investigating his brother's ties to Libya.

In an afternoon announcement from the White House televised live by the three networks, the president said he wants to appear before the subcommittee "at the earliest oportunity," adding, "the sooner the better."

Carter said a detailed White House report with supporting documents on the case, requested by the subcommittee yesterday, will be submitted to the Senate panel and will be made public early next week.

Carter said that once the report is made public he will hold a nationally televised news conference to answer questions on the case, even if the subcommittee is not prepared at that time to question him.

That will almost certainly be the case next week, according to the subcommittee's chairman, Sen. Birch Bayh (D-Ind.).

"I don't see how we could refuse to hear the president if he voluntarily wants to come forward at a given time," Bayh said yesterday. But he added that he did not think the subcommittee should seek his testimony "before all the evidence and all the facts have been accumulated."

The president's announcement, the first time he has discussed the case in public, was the White House response to a deluge of requests for records bearing on the Billy Carter matter from the special nine-member Senate subcommittee.

Bayh's subcommittee asked the White House to produce by Friday records and other information relating to Libya's attempts to influence U.S. policy, Billy Carter's relationship with the government of Libya, the Justice Department's investigation of that relationship and contacts between the White House and the Justice Department relating to the investigation.

Similar requests were made by the subcommittee yesterday to the State Justice, Energy and Commerce department, and CIA, the National Security Agency and the FBI.

White House press secretary, Jody Powell said he expects that most of the government agencies will have their reports ready for the subcommittee about the same time the White House report is submitted to the panel early next week.

Powell also said he "assumes" that all unclassified documents bearing on the case would be made public along with the report. He said this would include the written item the president found in his records of a "brief discussion" of his brother's case with Attorney General Benjamin R. Civiletti on June 17.

After days of assertions that there had been no contact between the White House and the Justice Department on the Billy Carter case, Civiletti last week acknowledged, after the president found the record of the conversation, that it had occurred, according to White House officials.

Carter told a group of House members last night that he will not ask for Civiletti's resignation and does not expect him to resign, according to Rep. Mary Rose Oakar (D-Ohio), one of about 22 junior Democrats at the White House meeting.

A question that was not answered by yesterday's announcement is whether the president is willing to appear at an open, nationally televised hearing on Captiol Hill. Powell said the precise "mechanism" for hearing Carter will be discussed with the subcommittee after the panel formally asks to question the president. He sais the White House did not anticipate difficulty in reaching agreement with the subcommittee on the mechanism.

The Billy Carter case became a major political problem for the White House after the disclosure earlier this month that the president's brother had accepted $220,000 in what he characterized as loans from the government of Libya.

After a lengthy Justice Department investigation, Billy Carter reluctantly agreed to register as an agent of the Libyan government, as required by law. Since the disclosure of the loans in the registration statement -- a characterization the Justice Department disputes -- the White House has been besieged with questions about possible interference in the Justice investigation and about the president's knowledge of the extent of his brother's ties to a foreign government.

The controversy has thrown Carter and his aides on the defensive, and could hardly have come at a worse time for them. It has added fuel to the effort by Sen. Edward M. Kennedy (D-Mass.) and other Democrats to deny Carter renomination at the Democratic National Convention, which convenes in New York City in less than two weeks.

Yesterday, the White House launced a political counteroffensive with the clear intent of smothering the controversy before the convention with an all-out display of presidentital candor and openness. While it may be weeks before the subcommittee is ready to hear from him, the president clearly hopes that the news conference he will hold on the subject next week will quiet public agitation over the case and diminish the importance attached to any later hearing conducted in the Senate.

Carter's appearance before television cameras in the White House press briefing room yesterday was his opening move to take the offensive.

"I have no doubt," he said, "that complete disclosure of the facts will clearly demonstrate that at no time did my brother influence me in my decisions toward libya or the policies of this government concerning Libya, and I am convinced that the facts will make clear that neither I nor anyone acting in my behalf ever sought to influence or to interfere in the investigation of my brother by the Department of Justice."

Discussing the subcommittee's plans yesterday Bayh said he would like to call Civiletti as one of the subcommittee's first witnesses next week, but he acknowledged that this may not be feasible.

Civilietti is scheduled to leave Thursday for an American Bar Association convention in Hawaii and go from there to Australia.

At its first formal meeting yesterday afternoon the Senate subcommittee authorized Bayh and Vice Chairman Strom Thurmond (R-S.C.) to search for a prestigious outside counsel to direct the investigation. For the moment, it is relying on Senate legal counsel Michael Davidson and deputy legal counsel Robert Kelley.

Sen. Bob Dole (R-Kan.) suggested that the hiring of a chief counsel come first, but Bayh said he felt strongly that the investigation should begin forthwith with preliminary hearings.

Bayh pointed out that the Senate Watergate Committee didn't begin its hearings untill more than three months after that investigation was authorized in 1973 because it was preoccupied with staffing problems. The Church committee that investigated the CIA and the FBI in 1975-76 took almost three months to hold its first hearing.

"If we follow that pattern," Bayh said, "we're saying that we're not going to have any investigation until the election is over. I don't think that's tenable at all."

Dole said he still had reservations about calling any major witnesses before a chief counsel is selected. He said the subcommittee needs "someone with impeccable credentials" whose reputation would enhance the credibility of the investigation.

The subcommittee's membership was rounded out of the meeting with the addition of Sens. Claiborne Pell (D-R.I.) and Richard G. Lugar (R-Ind.) as the Senate Foreign Relations Committee's representatives. The other seven senators, all appointed last week, are from Senate Judiciary.

The subcommittee is to meet again Thursday in an effort to set up a schedule of preliminary hearings for next week. Thurmond suggested that the panel might begin Monday with witnesses from the State Department, the CIA and other agencies to discuss relations with Libya.

In its request to the State Department, the subcommittee asked for all documents concerning any Libyan efforts to promote its interests in the United States, involving Billy Carter or anyone else, over the past 10 years. It also asked for a rundown of all State Department contacts with Billy Carter and communications about him.

The CIA was asked to provide records showing which officials in the White House or at the Justice Department had access to intelligence reports about Billy Carter's dealings with the Libyans and about Libyan efforts to obtain influence in this country.

The subcommittee made a similar request of the top-secret National Security Agency. The Commerce Department, meanwhile, was told to compile documents concerning export licenses for sales to Libya, along with a report on Libyan efforts to drum up business with American firms.

The Energy Department, in turn, was asked for information about oil exports from Libya and records concerning approached Charter Oil executives last year with a proposal to get the Libyans to increase their oil allocation for that company.

The subcommittee also issued a subpoena to Charter Oil for documents concerning its dealing with Billy.

"There are a lot of currents out there," Bayh told reporters. "Some people want to hang him [the president]. Some want a whitewash."

Saving he was determined to be fair, he said he thought preliminary hearings next week important so that no one at the convention next month can say that "we are speeding up or slowing down in order to affect the convention."

Bayh and Thurmond also pledged to pursue allegations that some members of Congress may have taken contributions from the Libyans. "If we're going to get involved in reports of Libyan influence-peddling, we've got to look under our own beds as well," Bayh said.

The senators, however, seemed unimpressed by fugitive financier Robert L. Vesco's claims over the weekend -- from his refuge in the Bahamas -- that he orchestrated Libya's payments of $220,000 to Billy Carter. Sen. Dennis DeConcini (D-Ariz.), who interviewed Vesco with Sen. Orrin G. Hatch (R-Utah), said Vesco openly stated that "his motive in getting the Libyans to pay Billy Carter is vengeance toward this administration."

Bayh said he was inclined to regard what Vesco said as "a cock and bull story." He said Vesco has been trying to destroy the president for months.

DeConcini has been authorized to conduct a separate investigation of Vesco's alleged connections with past and present White House aides, but has yet to decide whether to launch a full-blown investigation. The Carter administration has been seeking to return Vesco to the United States to face trial on long-standing criminal charges.

In Nassau, Vesco yesterday questioned the weekend accounts by DeConcini and Hatch, but refused to specify what parts he disagreed with.

In an interview at The Nassau Tribune offices, Vesco said his interview with the senators was in "no way intended to be an attack on President Carter or anyone else, but simply to respond to questions accurately and truthfully as they were posed, letting the chips fall as they may, without in any way trying to judge the culpability of any person.

"From published reports so far, it would appear that the reelection atmosphere is resulting in a series of statements being attributed to me, and such statements are not necessarily accurate."

(He declined to say which statements were inaccurate, and also refused to discuss Billy Carter.

The controversy over Billy Carter also reverberated in the House, where the Foreign Affairs Committee voted to recommed formal adoption by the full House of a resolution in inquiry.

In a letter to the committee, the president promised to supply the documents listed in the resolution by Aug 18. Several committee members protested that approval of the resolution would smack too much of a confrontation with the White House when the president had already pledged cooperation. But in the end, only Chairman Clement J. Zablocki (D-Wis.) voted against the committee's action.

The chief sponsor of the resolution, Rep. Robert E. Bauman (R-Md.), had hoped to get it to the House floor by Friday -- when the House will recess for the Democratic convention. But Zablocki said he had no intention of bringing it up until the convention is over.