Former president Jimmy Carter said here today that President Reagan's support of the government of El Salvador was underwriting a regime that was "the most bloodthirsty in our hemisphere, perhaps in the world" and he accused the administration of taking "the leadership role" in trying to overthrow the Sandinista government in Nicaragua.

Carter said the Reagan administration is spending "enormous amounts of money" on the anti-Sandinista campaign, despite official denials and legal restraints imposed by Congress. He said the policy would encourage the Nicaraguan leaders to rely on Cuba and the Soviet Union as their only sources of support.

In a news conference, Carter, who is on a six-day private visit to Japan, also made his most extensive remarks to date on the briefing papers prepared for his 1980 debate with Reagan. He said that the papers "incorporated the very essence" of his presidential campaign and that illicit access to certain key materials by Reagan supporters would have greatly benefited his opponent's campaign.

He said the comprehensive papers contained "the issues that we had identified through secret polling and . . . our own awareness of the American political scene as being the most crucial and important," key portions of which were withheld from even top Carter campaign officials.

"Whether he Reagan had all this material I do not know," Carter said. "If he did, it was obviously of great benefit to him." But he stressed, "I am not prejudging what he had."

While guarded on the subject of the briefing papers, Carter was sharply critical of Reagan's handling of foreign policy. He made particular reference to Central America, where he said Reagan had sharply reversed Carter's own approach stressing human rights and democratic reforms.

Blaming the Salvadoran government for the deaths of 30,000 to 35,000 of its own people, Carter said that his administration's policy had been to back democratic factions in Latin America, while allowing the region's democracies to mediate conflicts.

In response to criticism of its strong military backing of El Salvador's provisional government, the Reagan administration at times has argued that its policy is a continuation of Carter's strategy of combining military assistance with pressure for reforms there.

"President Carter did not hesitate. He authorized arms and munitions to El Salvador," Reagan said in an April speech apparently referring to Carter's decision to send $5 million in emergency military supplies there three days before he left office.

That decision, in response to a major guerrilla offensive, came six weeks after Carter had suspended all military aid to El Salvador following the alleged involvement of Salvadoran soldiers in the slayings of four American churchwomen working in the country.

Carter blamed Reagan's "unprecedented radical departure" in foreign policy, including a recently agreed-upon increase in U.S. arms sales to Taiwan, for creating "a great deal of doubt" among the leadership in China about the true intentions of U.S. policy on that country. He said, however, that Reagan was "at least now making some overtures toward correcting the early damage to U.S.-China relationships."

Carter said it would not be appropriate for him to anticipate the results of investigations by the Justice Department and a congressional committee into the Reagan campaign's possession of Carter documents. He indicated, however, that he believes that more than one of his staffers was involved in leaking the information.

He said the debate briefing materials were "tightly held within a few people in the White House itself." He added that "there were no copies of this material ever made available to our campaign headquarters."

In addition, he noted, Richard V. Allen, President Reagan's former national security adviser, has said that other material came "directly from the National Security Council" in the Carter White House.

From this and descriptions of the papers that have been turned over to the press and the Justice Department, Carter said, it was obvious to him that "there were a whole series of losses of papers from the White House, not just one batch."

But Carter stressed that he was trying to be "as unsubstantive as possible" in responding to questions about the matter today because he did not know just what materials had actually reached Reagan and what use was made of it.

"I don't know anything about it as far as who took the materials, how many times they were taken, how many people were involved," he said. "I don't know who received the materials, I don't know how much benefit was derived by the Reagan campaign from the material. The only thing I know about is what I've learned from the news media itself."

Carter said he was surprised at the "longtime involvement" of some leading members of the American press who had knowledge of the incident, but failed to make "any revelation to the public."

Asked about the apparently more prominent role of spying in political campaigns in recent years, Carter said, "I don't think it's proper for a campaign to pilfer documents of this kind . . . . It's not a normal or accepted policy in the political society of our nation. I think examples of this in the past to be quite rare."