The United States and the Soviet Union opened negotiations today for the first time in 15 months to seek an agreement that would curtail nuclear weapons as well as prevent an arms race in space.

Soviet negotiator Viktor Karpov told reporters that the new Soviet leader, Mikhail Gorbachev, was closely involved in the preparations for the arms talks and, in fact, "presided over the Politburo meeting that approved the instructions last Thursday." This was seen as an indication that the death Sunday of Soviet president Konstantin Chernenko would not block the progress of the talks.

The arms talks, perhaps the most complex ever undertaken by the two superpowers, were launched in a highly charged political atmosphere as two teams of congressional observers hailed the event as a "historic opportunity" and vowed to play an active role in pursuing the search for "a more stable and secure strategic balance."

Sen. Richard Lugar (R-Ind.), the chairman of the Senate Foreign Relations Committee, said the presence of 10 senators and eight congressmen was intended to show there would be "vigorous participation" by Capitol Hill because of the "importance of forming a bipartisan consensus" behind any future arms control agreement.

Sen. Sam Nunn (D-Ga.) said the Senate had failed to ratify three key arms control treaties in 12 years and needed to be closely involved from the start of what are bound to be long and complicated negotiations.

The arms talks began today with a "ground rules" session including a discussion of the agenda and procedures for the three sets of negotiations covering strategic or long-range nuclear arms, medium-range nuclear missiles and space-based systems.

At 11 a.m., the U.S. delegation, headed by Max Kampelman, who will also serve as negotiator on space arms and defensive systems, drove past a flock of journalists and antinuclear campaigners into the Soviet mission. Kampelman was accompanied by fellow negotiators John G. Tower, responsible for strategic nuclear arms, and Maynard W. Glitman, in charge of medium-range weapons.

After signing a book of condolences for Chernenko, the U.S. team was escorted by Karpov, the leader of the Soviet delegation, into the same room where the SALT II agreement on limiting nuclear strategic weapons was negotiated.

Karpov, who will deal with strategic arms, was not joined by other members of the Soviet negotiating team, Yuli Kvitsinsky for space arms and Alexei Obukhov for medium-range nuclear weapons. U.S. officials speculated that Karpov's solo appearance could have been a gesture to indicate Soviet insistence on a "package deal" linking progress in all three sets of talks.

"I hope that our meeting will not be the last one, but one of a first in a series in which we will negotiate and reach an agreement," Karpov told Kampelman upon his arrival.

"Our objective is to reach an agreement too," the chief U.S. delegate said in response. "I hope this is a good omen."

Following the meeting, which lasted 2 hours and 45 minutes, Kampelman read a statement saying the two sides held "a serious and businesslike discussion of the issues" and agreed to resume the talks Thursday at the U.S. arms control mission here.

Kampelman declined to answer any questions, saying the delegations had approved a rule of confidentiality to restrict public discussion of the contents of their secret talks.

"The nature of these negotiations is such that in our view we should be talking to our Soviet colleagues and not to the public," he said.

Earlier, however, while waiting for the American delegation to arrive, Karpov freely answered reporters' questions.

Asked if he would return to Moscow for Chernenko's funeral, Karpov stated, "No, I'm not going. I'm here to negotiate."

Regarding the controversial topic of space-based weapons systems, Karpov said, "everything is negotiable if you want that. If there is a real interest in negotiating a ban on space arms, it can be done. And the more we do now to prevent the arms race in space, the better."

He admitted that verification poses a problem for any future treaty, but he said both sides were aware of the difficulty as well as the need "to make an agreement that can be verified by both sides, not only by the United States but by the Soviet Union."

Once substantive talks begin, sources close to the U.S. delegation said they expect the Soviet to propose a freeze on missile deployment while the talks are underway, if only to seize the initiative for public opinion.

Before the talks opened today, members of western peace groups met with senior officials at the Soviet Mission to urge a freeze on nuclear deployments and a ban on space research during the negotiations.

But the Americans, while conscious of the battle for public opinion in Europe, have ruled out the idea of moratoriums either for missiles or space-based research, the sources said. The United States and its allies are still intent on proceeding with deployment of Pershing II and cruise missiles in Europe through 1988, and Belgium and the Netherlands are facing difficult political tests over taking 48 cruises apiece this year. In addition, an important test of a U.S. antisatellite device is expected to take place soon.

The United States is expected to counter the Soviet arguments against the risks of space research by saying Moscow is pursuing a clandestine space defense program of its own.

In addition, the Americans will rebut any Soviet charges that U.S. military programs in space could jeopardize the 1972 antiballistic missile treaty by stressing that the phased-array radar system being constructed at Krasnoyarsk will become a violation when it begins operating.