In a subdued manner befitting the near-hopelessness of the task, the U.N. General Assembly opened a special session on disarmament yesterday that will seek to devise new ways of slowing the $400 billion global arms race.

General Assembly President Lazar Mojsov of Yugoslavia, in opening the six-week session, conceded that although the United Nations has adopted 228 resolutions on disarmament since its creation in 1945, "No serious breakthrough has been made."

Nevertheless, Mojsov urged delegates from 149 nations to make this special session - which is being billed as the largest disarmament conference in history - "a genuine and unmistakable turning point" in world efforts aimed at arms control.

Tight security measures were in effect as the special session, which will bring more than 20 presidents and prime ministers to New York, got under way.

But U.N. hopes of turning the six-week conference into a media spectacular that would focus world attention on disarmament seemed considerably diminished by the decisions of President Carter and Soviet President Leonid Brezhnev not to attend.

Vice President Walter Mondale will present the U.S. proposals for the session in a speech today, and Soviet Foreign Minister Andrei Gromyko will address the assembly on Friday.

The opening-day atmosphere yesterday distinctly lacked the tension and excitement that have surrounded major debates in the past, or the crowds of demonstrators that have turned out for appearances here by such figures as Fidel Castro and Yasser Arafat.

The small cluster of 50 pro-disarmament demonstrators that gathered across the street from the United Nations - led by 10 Japanese monks in orange robes and beating tom-toms - was outnumbered by police by perhaps 10 to one.

U.N. Secretary General Kurt Waldheim, the other first-day speaker, urged delegates to the session to chart a strategy for disarmament, establishing principles, priorities and long-term goals.

He suggested that the assembly set up an international advisory board of experts in the field of arms control to guide U.N. arms control efforts. He also proposed that all nations should devote at least $1 million to disarmament efforts for every $1 billion they spend on arms.

A number of far more ambitious arms control schemes are expected to be presented during the course of the special session.

French President Valery Giscard d'Estaing, who is scheduled to address the assembly Thursday, is bringing with him a complex variety of proposals that include orbiting U.N. space satellites to monitor world disarmament measures.

The General Assembly, however, is not expected to have time to give much more than initial consideration to these proposals during the six-week session.

Experts feel the most that is likely to come out of the meeting is a declaration on disarmament and a resolution setting out a future course of action.

One possibility that concerns U.S. officials, however, is that the assembly might move to create a new U.N. disarmament negotiating forum to replace the 31-member Geneva conference of the Committee on Disarmament.

The Geneva conference has been the primary negotiating vehicle for disarmament treaties since the 1960s, but a number of nations have long been unhappy over the Soviet-American preference to work out their agreements in private - and then present them to the Geneva forum.

Other countries like France, which pulled out of the Geneva conference during the DeGaulle era, resent the fact that the United States and the Soviet Union serve as permanent cochairmen of the Committee on Disarmament.

France is expected to press for U.N. establishment of a successor negotiating conference that would have rotating cochairmen - a feature that might also appeal to China, which has chosen not to take part in the Geneva proceedings.

U.S. officials suggest privately that they might consider agreeing to a rotating chairmanship for the Geneva conference, but say they are flatly opposed to creation of a new forum.

"We are not prepared to abandon the conference of the Committee on Disarmament," a top U.S. official said. "This is a forum in which progress can be made."

In addition to Giscard, other world leaders planning to address the special session include West German Chancellor Helmut Schmidt, British Prime Minister James Callaghan, Italian Prime Minister Giulio Andreotti, Canadian Prime Minister Pierre Elliot Trudeau, and Indian Prime Minister Morarji Desai.

While Brezhnev's decision not to come to New York is largely excused here for reasons of health, many delegates are puzzled by the absence of Carter - particularly in view of his inaugural pledge to work for "elimination of all nuclear weapons from this earth."

Some experts suggest, however, that with both the SALT talks and a variety of other U.S.-Soviet arms negotiations at a delicate stage, the president may have decided it would not be wise to appear to be trying to outbid the Soviet Union in what is essentially a propaganda forum.

Consequently, U.S. officials are awaiting Gromyko's speech on Friday with considerable interest. They expect the Soviets will try to score the normal number of propaganda points, but are hoping Moscow will not, for instance, introduce a resolution calling for outlawing the neutron warhead.

While President Carter has temporarily postponed production of the neutron warhead, he has said the final U.S. decision whether to manufacture the weapon would be "influenced by the degree to which the Soviet Union shows restraint in its conventional and nuclear arms program."

"There's no doubt that the Soviets will make a propaganda issue of the neutron bomb," a U.S. official said, "but we hope they will not make it a major issue."

While the disarmament discussions are occupying center stage, the special session is providing an opportunity for more than 50 foreign ministers to hold private hotel-room talks this week on issues like Africa and the Middle East.

Secretary of State Cyrus Vance arrived in New York yesterday and will spend most of his time engaged in bilateral talks before returning to Washington on Thursday.