President Reagan said yesterday he is "most interested" in Soviet President Leonid I. Brezhnev's invitation to a summit conference, but indicated that several conditions would have to be met first.

Among these, the president said, are that the Soviets straighten out the issue of their arming of guerrilla movements in El Salvador and elsewhere, and that any negotiations consider the entire range of U.S.-Soviet relations.

"I think that you can't just deal with just one facet of the international relationship, you've got to deal with all of the problems that are dividing us," Reagan said in a brief question-and-answer session with reporters.

Reagan said that he wanted to discuss the Soviet suggestion with his advisers and with U.S. allies, starting with British Prime Minister Margaret Thatcher, who visits Washington this week. "I have pledged to [the allies] that we're not going to act on things like this unilaterally," Reagan said.

High administration officials, as well as visiting French Foreign Minister Jean Francois-Poncet, continued to react positively, though somewhat warily. Brezhnev's unqualified call for Soviet-American dialogue at all levels was considered surprisingly conciliatory in view of the hard line the Reagan administration has taken against the Soviets.

Although Brezhnev called for a summit meeting, he did not specify negotiations on any specific issues. Reagan reiterated his long-standing position that he will negotiate with the Soviets "if it's a legitimate negotiation aimed at verifiable reductions [of] strategic nuclear weapons."

Brezhnev invited a summit meeting during his speech to the opening of the 26th Congress of the Soviet Communist Party Monday. The suggestion came out of the blue, according to White House press secretary James S. Brady. He said there had been no private communication between Washington and Moscow about a summit, as would be usual before going public with plans for the leaders of the superpowers to meet.

Although Reagan stressed the importance of linkage and suggested that there would be some U.S. conditions before a summit could be arranged, he left himself some flexibility by saying, in reply to a separate question, that a summit agenda would not have to be so complete that the Soviets had already agreed to do things the United States wants.

Reagan was asked what the Soviets would have to do about the shipments of arms to El Salvador "in order to qualify for such a summit conference."

The president noted that the Soviet Union has denied being involved in the arms shipments, but said the United States has information which makes "it evident that [the Soviets] are involved."

He added: "I would think that this would be one of the things that should be straightened out -- their participation in that kind of activity."

American and French sources found Brezhnev's indication of willingness to extend a zone of "confidence-building" military measures as the most intriguing hint of a Soviet concession.

At present, the Soviet has agreed to advance notification, and international observation, of military maneuvers within 250 kilometers of the western border. France has insisted that this zone be extended to all the Soviet Union west of the Ural Mountains, in connection with the French-sponsored plan for a European Disarmament Conference. In the past, Moscow has refused, according to French sources.

Reagan made his unexpected appearance before reporters who had been listening to Education Secretary Terrel H. Bell and Health and Human Services Secretary Richard S. Schweiker discuss the proposed federal spending cuts.

The president began with a prepared statement calling the reaction to his tax and spending proposals "enormously encouraging." He reiterated his appeal for bipartisan support in Congress.

The brief question period permitted dwelled entirely on foreign affairs.

The Reagan administration has been escalating statements about its concern and likely response regarding Soviet arms to El Salvador, and has also taken a rhetorically tough stance on bilateral U.S.-Soviet relations.

The president was asked whether there is any danger that the United States, which is considering increasing its contingent of military advisers in El Salvador and has refused to rule out a blockade or other actions against Cuba through which a large part of the Soviet arms pass, might become so involved in El Salvador that if found it hard to get out.

"No, I don't think so. I know that that is a great concern. I think it's part of a Vietnam syndrome. But we have no intention of that kind of involvement," the president said.

Despite that statement, however, Reagan said there should be no doubt that "we are in support of the government there against those who are attempting a violent overthrow of the government."

Reagan said he would not intentionally be dragging his feet on the possibility of a summit, but there would be lengthy preparations. He said, "the world is going to see that this isn't something that you just say, 'Well, you know, come on over, let's talk.'"