Soviet President Leonid Brezhnev opened his long-delayed visit to West Germany yesterday with a call for a "deepening detente" between East and West.

Arriving here on his first foreign trip in nearly a year, the 71-year-old Soviet leader reaffirmed interest in an agreement "to prevent the birth of the neutron bomb," and called for new steps to reduce arms and armies "especially in central Europe."

But Brezhnev's frail health was a major factor hanging over his four-day visit here. He came down the steps of his special Ilyushin-62 airliner with the hand of a military aide resting on his arm. Later in the day he had difficulties getting up from his seat at the end of a session with photographs.

Brezhnev's West German hosts have scheduled lengthy rest periods in his schedule, which has alreday been pared down to remove the planned trip to the Ruhr region and appointments with various industrial leaders.

Two Soviet vans carrying special heart and lungequipment also were flown from Moscow to Bonn and to Hamburg, where Brezhnev will spend the final day of his visit at the home of Chancellor Helmut Schmidt, German sources said.

Despite his frail health, the Soviet leader seized on this visit to mount a new diplomatic initiative. His statements here take on added significance because of West Germany's central role in NATO.

It is on West German soil that neutron weapons would be based if President Carter eventually decides to produce them, and the Soviets have carried on a heavy campaign to pressure Bonn not to accept these controversial weapons.

Chancellor Schmidt is also a central figure in Allied efforts to break the long-stalled talks over arranging mutual East-West troop reductions in central Europe, and Moscow is seeking to influence the German chancellor to press for more favorable Allied proposals.

It is widely felt here that Brezhnev will try to exploit the difference between Bonn and Washington, but officials say there is no chance for the Soviet leader to split the NATO partners on questions of mutual security.

West German and Soviet spokesmen said last night that the initial meetings between the two leaders were "open, congenial and concrete" and that the desire to cooperate was very strong on both sides."

The Soviet leader's remarks on armaments came during a speech at a state dinner given in his honor last night by West German President Walter Scheel.

Brezhnev said that German-Soviet relations had become "enriched, important amd sizable" as a result of the dramatic Moscow-Bonn reconciliation treaty of 1970 that helped heal the bitter wounds of World War II.

He said millions of Germans and people elsewhere wanted peace, "but we also know that there are otehrs in the West and also in Germany who don't want detente." Those who "believe that Soviet Union has malicious intent are wrong," he said. Thus mistrust, he added, is what breeds the arms race.

Without yielding any details of what he had in mind, Brezhnev called for a new global arms agreement that also would cover Europe and especially central Europe.

"Let us agree to relinquish production and deployment of new systems of mass battle weapons and through this mutual agreement. close out the birth of the neutron bomb" - a weapon which he said the United States "wanted to transfer to the peoples of this continent like an ominous gift."

In remarks that may well reflect his own feeling of failing health, Brezhnev told guests at the dinner that "one has to deepen detente to make it irresversible."

"During war, one gets used to danger and in good times to prosperity. As long as one isn't sick," he said, "You get used to good health. For more than 30 years now, there is peace in Europe. That has never happened before. People have started to get used to peace as if it were something natural, as if detente isn't subject too many trials."

One of those trials, however, is deep Western concern over the cintinuing buildup of Soviet tank armies and nuclear-tipped rockes aimed at central Europe. Schmidt is expected to stress those concerns to Brezhnev. He is also expected to express Western expectations for a more substantial Soviet response to President Carter's deferral of neutron weapon production than a simple Soviet abstention on the same weapon. The neutron warhead is meant to defend against tank in central Europe than the NATO countries.

DSchmidt has already said that Bonn will accept deployment of neutron weapons if they cannot be covered by an arms control agreement and if NATO approves deployment.

At the dinner, Brezhnev heard what amounted to a plea from German President Scheel to turn off for ever the Communist propaganda that is still sometimes directed at Germany about the Nazi past and the alleged rise of neo-Naxism.

Such criticism, Scheel seemed to be saying, is inconsistent with relations that have steadily improved in both diplomatic and economic sectors since the late 1960s. "There is no room for hate and jealousy against other nations," Scheel said.

He thanked Brezhnev for efforts thus far in allowing some ethnic Germans in teh Soviet Union to return, though Bonn is privately still unhappy that many more who want to come have not been allowed to leave.

The emigration issue actually is one of several in the field of human rights that has led to protests marking the Soviet leader's visit.

The biggest human rights demonstration is et for today which a march through Bonn is to be led by a leading dissident, former Soviet Maj. Gen. Pyotr Grigorenko. Security is tight, with an estimated 20,000 police and special military units involved.

The four days of talks are expected to cover detente, disarmament, and economic matters. Though German officials are stressing that no major new political agreements will be signed except an extension of an economic cooperation treaty, the visit has also taken on special significance because it come before key NATO and United Nations meetings on disarmament later this month.

Brezhnev's visit to Bonn was first scheduled for the fall of 1976 and was postponed several times. His last trip outside the Soviet Union was to Paris last June.