Soviet President Leonid Brezhnev arrived here today on an official visit, putting his personal prestige behind the Kremlin's effort to entice Western Europeans into rejecting new U.S. nuclear missiles planned for Europe.

Touching down at the airport here in the early evening, Brezhnev came late enough to miss tens of thousands of demonstrating West Germans, Afghans and Soviet exiles who had massed in downtown Bonn to protest Moscow's arms policies and the invasion two years ago of Afghanistan.

The focal point of the four-day visit, during which Schmidt and Brezhnev are scheduled to meet formally three times, including once without aides, will be how to curb European-based nuclear weapons. Schmidt and other West European leaders face substantial public opposition to North Atlantic Treaty Organization plans to station new Pershing II and cruise medium-range nuclear missiles in their countries.

The two leaders rode together from the airport to Gymnich Castle, 30 miles outside Bonn, where Brezhnev is staying. West German television showed them, with West German Foreign Minister Hans-Dietrich Genscher and Soviet Foreign Minister Andrei Gromyko, sitting down for drinks in the castle.

Schmidt told reporters afterwards that arms reduction had been the central theme during his first conversation with Brezhnev. He said there was "real pleasure on both sides" at seeing each other again.

A formal welcome will be held Monday, before the first round of talks.

At the airport tonight to greet Brezhnev, who will be 75 next month, were several thousand members of the West German Communist Party and other leftist groups. They carried banners declaring that Brezhnev had come on a mission of peace and good will and opposing nuclear weapons and nuclear bases, meaning U.S. bases in West Germany.

This is Schmidt's sixth meeting with Brezhnev -- the most of any major Western leader. In preparing for the visit, Schmidt consulted with other key Western alliance officials, including British Prime Minister Margaret Thatcher, who was in Bonn last week, and President Reagan, with whom Schmidt spoke by phone last night.

With U.S.-Soviet negotiations on weapons set to begin Nov. 30 in Geneva, Schmidt has said he would explain the Western position to Brezhnev and search for common East-West ground. Reagan's offer last week to cancel the U.S. deployment if the Soviets agreed to dismantle their new and old medium-range, land-based missiles has strengthened Schmidt's hand in the talks here and will be the basis for his conversations with Brezhnev. The Soviets have termed Reagan's proposal a propaganda ploy.

The visit -- Brezhnev's first to the West since the invasion of Afghanistan and only his third trip abroad since then -- has obvious advantages for the Soviets, who are expected to use West Germany as a platform to further their campaign against U.S. deployment.

A paper on Soviet and Western military forces that was printed in West Germany but prepared by Soviet sources and dedicated to Brezhnev's visit has been distributed here to bolster Soviet claims that the new U.S. weapons would upset an East-West nuclear balance in Europe.

The Bonn government's chief spokesman, Kurt Becker, labeled the paper an attempt at "disinformation" aimed at "bringing uncertainty into the debate" about nuclear weapons. It is seen here as the Soviets' counter to a U.S. booklet, "Soviet Military Power," published last month.

The weekly Die Zeit described Brezhnev's visit as "targeted in many respects beyond Bonn at the entire Western public opinion, the governments and the peace movements."

West Germany's nervousness about its own exposed position showed up last week in the labored efforts by Bonn government officials to find a proper characterization of Schmidt's role in the talks. Becker dismissed such terms as "broker" or "mediator," settling finally on calling the chancellor an "interpreter."

Several demonstrations against Brezhnev's visit were held here over the weekend. About 5,000 Afghans came from all over West Germany to march today, chanting "Death to Brezhnev" and "Russians out of Afghanistan."

About 12,000 supporters of the West German leftist party The Green demonstrated for a European nuclear-free zone, and several thousand pro-Soviet Marxists marched to a chant of, "What Hitler did not achieve, Schmidt is doing with NATO power."

The rally that drew the largest crowd, between 30,000 and 50,000, according to police and organizers, was promoted by West Germany's conservative Christian Democratic and Free Democratic parties.

Speakers including the leader of the Christian Democrats' youth wing, Matthias Wissman, the Free Democrats' security expert in parliament, Juergen Moellemann, and exiled Soviet dissidents Vladimir Bukovsky and Vladimir Maximov called on the Soviet Union to pull out of Afghanistan, stay out of Poland, and accept Reagan's offer of a "zero solution" for European-based medium-range nuclear weapons.

Although it drew a larger turnout than expected, the protest amounted to only a fraction of the quarter of a million who massed in Bonn last month to protest the U.S. missiles plan.