U.S. British and French leaders today issued an implied warning to the Soviet Union that East-West "detente would be seriously threatened" by violation of four-power agreements covering the divided city of Berlin.

The warning came in the form of a declaration that the leaders of the three World War II Western Allies issued during a meeting here after the seven-nation economic summit meeting just ended.

Although Allied reaffirmations of their rights in Berlin have been made periodically, today's statement carries added significance.

For one thing, it is rare for such declarations to be issued directly by the heads of government, in this case President Carter. French President Valery Giscard d'Estaing and British Prime Minister James Callaghan.

In addition, several Soviet and East German moves in recent months have been interpreted in the West as efforts to change the status of the Communist-ruled eastern sector of Berlin unilaterally and gradually erode Western status there.

Thus far, the allies havea had no success in combating those moves.

In January, the three Western powers delivered a protest to the Soviets, claiming that such things as new one-day visa requirements for non-German visitors to East Berlin and various changes in border-control points were violations of the 1971 four-power agreement on Berlin among the Allies and the Soviet Union.

Informed sources say the Soviets have privately rejected that protest, but neither side has made a public statement on the fate of the protest since January.

The 1971 agreement states that "the situation that has developed in the area (Berlin) . . .shall not be changed unilaterally" by any of the four powers. But although the agreement has worked well in many respects, it is vague, and the East Germans and Soviets have long tried to argue that it applies only to West Berlin and that East Berlin is in fact part of East Germany and is its capital.

Today's statement was especially welcomed by the West German government. Bonn views it as a strong commitment by a new American President to the survivial of West Berlin and to continued rights and access in East Berlin.

West German Cahncellor Helmut Schmidt, who sat in on the Berlin meeting told reporters as he was leaving the 2-hour session here that the declaration "was a very good one."

President Carter also said the meeting went very well, but he was also the source of some befuddlement here last night when he appeared to describe the situation in divided Germany incorrectly.

During a briefing session with reporters last night, Carter spoke movingly of the Berlin Wall as a "very dramatic indication of the hunger for freedom among people who live in East Germany, I don't know how to express any hope that it might be removed."

Just before that, however, Carter had said in response to a question: "We maintain, as you know, American, French, British and German, West German patrols in Eastern Germany, and according to the agreement, the East Germans have access to Western Germany as well."

Asked about this today, a West German patrols in East Germany and no East German acess to West Germany that he knew of. But he quickly added that the Bonn government "is in no way disturbed about this," meaning Carter's remarks.

In the joint declaration on Berlin today, the three powers reaffirmed that the status of the "special area of Berlin could not be changed unilaternally." They said they would continue to reject all attempts to put into question four-power rights relating to Germany as a whole and the four sectors of Berlin, three of which make up West Berlin.

News agencies reported the following:

Presidents Carter and Giscard conferred on the world situation and problems in American-French relations. Spokesmen for both sides described the meeting as cordial, saying that the two leaders are on excellent personal terms.

Carter who called it "a very fine meeting," told reporters that he had accepted an invitation to visit France and disclosed that he had personally backed the French initiative last month to help Zaire's government in its struggle against invaders. A French spokesman said Carter had expressed his approval of the Zaire operation in a cable to Giscard April 15.

U.S. officials said Giscard had surprised Carter by not bringing up the subject of landing rights in New York for the supersonic Concorde airliner - one of the most barbed issues in U.S. French relations.

The breakfast seemed to dissolve any suggestion that Giscard had snubbed President Carter when he refused to attend a dinner before the seven-nation summit meeting. The French say that Giscard stayed away from the dinner because it also included European Commission President Roy Jenkins, whom they do not recognize as ranking with heads of state or government.

No date was set for Carter's visit to France, but it was announced that French Prime Minister Raymond Barre, in response to an invitation from Carter, plans to visit the United States in mid-summer.