Almost 2 1/2 years after taking office, President Carter finally met Soviet President Leonid Brezhnev today, and both immediately agreed that is was long overdue.

In reporting the exchange on the eve of their first formal summit session Saturday, U.S. officials said both Carter and Brezhnev indicated that their next meeting should take place much sooner.

The two leaders met briefly while paying a joint formal call on Austrian President Rudolf Kirchschlaeger, official host for the U.S.-Soviet summit.

The 72-year-old Soviet leader arrived by air from Moscow earlier today to a formal greeting under sunny skies. He walked unaided but stiffly through the ceremonies, his demeanor alternating between lively interest and silent, stolid impassivity that underlined his weakened physical condition. Brezhnev wore his customary array of medals on his suit, three gold stars on his right side and three Hero of the Soviet Union medals and one Hero of Soviet Labor on his left side.

Unlike Carter yesterday, Brezhnev made no arrival remarks. Later today, Brezhnev honored the dead at both the Soviet war memorial and the Austrian memorial to the unknown soldier and attended the Vienna Opera with Carter. Both leaders left at intermission.

The Soviet leader uttered no word publicly at any of his three appearances, which were attended by crowds of reporters, Soviet officials, Austrians and tourists.

Brezhnev, who wore a hearing aid in his ear, seemed not to hear a reporter's query shouted at him from 15 feet away. Soviet officials later ruled out any questions from reporters at the war memorial in Schwarzenberg Square.

Brezhnev's infirmities have made their mark on the summit, which starts Saturday morning at the U.S. Embassy where the two men will take up the issues that divide the superpowers. They will sign the new Strategic Arms Limitation Treaty (SALT II) Monday.

American officials said the first session will be a largely formal exchange of views.Carter is to open by delivering from notes an overview of U.S. global policy. Brezhnev is expected to present general Soviet positions from a formal, written statement.

The afternoon meeting will center on the treaty itself and the mutual intention to proceed immediately to the complex problems of the follow-up SALT III negotiations, which will directly involve European-based forces for the first time.

Carter has been looking forward to this meeting since before he took office. But the Russians insisted that the SALT II treaty must be completed first. That look far longer than either side anticipated. It has been 4 1/2 years since the last SovietAmerican summit meeting.

American officials continued to emphasize the importance Carter attaches to the chance to state firmly, clearly and directly to the top Soviet leader and his aides the concerns of the United States and its allies about areas of superpower conflict. Not only the substance but the "strength with which those positions are held and the reasons for them" are to be made clear to Brezhnev, the official sources said.

Among the U.S. concerns that Carter will express are the operations of Soviet-backed Cuban troops in Africa, the buildup of Soviet nuclear and conventional forces in Europe and the Soviet strategic nuclear buildup, which is considered a potential threat to U.S. land-based intercontinental ballistic missiles.

The president wished to make the decision to move ahead with the new generation MX missile before the summit so he would be in a position to explain it to Brezhnev if requested to do so, according to U.S. sources.

The sources said Carter did not personally announce the decision on the new missile because this might have been seen by the Russians as provocative behavior on the eve of the summit meeting.

The Soviet delegation includes Foreigh Minister Andrei Gromyko, Defense Minister Dmitri Ustinov, and Brezhnev's protege Konstantin Chernenko, all members of the ruling Politburo. Not since 1968, when eight Politburo members went to Bratislava for a showdown with Czechoslovak leader Alexander Dubcek before the Soviet invasion, have so many top leaders left Moscow together for a summit.

Chernenko, 67, whose quick rise into the inner ruling circle in the past three years has marked him as a possible successor to the top post, was never far from his leader during today's outings. He occasionally offered advice and directions, as did Brezhnev's military adjutant.

The Soviets reportedly have brought in more than 60 elite security men to protect the leaders, who are staying in specially prepared suites at the Soviet Embassy in Vienna's old city center.

Brezhnev, a car buff who had his Rolls-Royce airfreighted here, used his usual Zil limousine today. But he indulged his affection for cars by riding in the front seat on the trip into the city from the airport.

Tonight, he seemingly gave in to his personal tastes again by walking out at the intermission of the premiere of a new production of Mozart's "Abduction from the Seraglio," despite the fact that it was specially shortened for the two visiting heads of state.

The audience in this city of opera lovers clapped politely when Carter and Brezhnev arrived, but reserved stormy applause for 84-year-old conductor Karl Boehm, one of Europe's most famous maestros, who was making a special guest appearance.

Just after Brezhnev left, Carter did the same, leaving behind the first lady and daughter Amy, together with Secretary of State Cyrus Vance to hear the opera through to its end. CAPTION: Picture 1, Presidents Carter and Brezhnev during their first, informal meeting in Vienna. AP; Picture 2, Austrian President Rudolf Kirchschlaeger is flanked by President Carter and Soviet leader Leonid Brezhnev at the Vienna State Opera last night. AP