President Reagan sent U.S. negotiators off to "long and difficult" arms talks in Geneva yesterday with instructions to discuss his "Star Wars" missile-defense program with the Soviet Union but not to negotiate new limitations on its research or testing.

In a brief White House ceremony, Reagan charged his new Geneva bargaining team to seek the earliest possible agreement on "real and verifiable reductions" in U.S. and Soviet offensive nuclear arms. The ceremony followed a private meeting in the Oval Office at which Reagan delivered his final instructions for the opening U.S. positions, including some new features in the position on long-range nuclear arms.

"We should have no illusions that this will be easy since any venture of this magnitude will take time," Reagan said publicly to his three chief negotiators, Washington attorney Max M. Kampelman, former senator John G. Tower (R-Tex.) and career diplomat Maynard W. Glitman. The negotiators and their aides left for Geneva before day's end.

The negotiations "will clearly be long and difficult" because vital security interests are at stake, Reagan said. "We're realistic because we know that our differences with the Soviet Union are great. Patience, strength and unity -- Western unity -- will, therefore, be required if we're to have a successful outcome."

Reagan mentioned no timetable for agreement, nor did his national security affairs adviser, Robert C. McFarlane, who briefed reporters on the United States' positions as it goes into the talks. Many official as well as nongovernmental observers say they believe that it will take years to produce substantial agreements, if such accords prove possible at all.

As agreed by the two sides in January, the negotiations will be in three parts: strategic offense or long-range nuclear arms, intermediate-range nuclear forces in Europe and space arms.

Reagan and McFarlane spoke of reductions in offensive nuclear arms as the top U.S. priority. Various statements from Moscow and remarks by a visiting Soviet parliamentary delegation here this week have made clear that limits on space arms, especially Reagan's Strategic Defense Initiative, or Star Wars, is the top Soviet goal.

Emphasizing their "interrelationship," Moscow has said progress in the space area will be necessary for agreements to be reached in the two other areas. Soviet officials have recently threatened to increase the already-massive Soviet offensive-weapons programs if Reagan goes ahead without restrictions on his strategic-defense program.

McFarlane, in discussing plans for the space talks, said Reagan's written instructions of more than a dozen pages call for discussion with the Soviets of "a comprehensive rationale" for the Star Wars plan as well as "the relationship between offense and defense."

The negotiators in the space talks will also present U.S. concerns on "the erosion" of the 1972 Antiballistic Missile Treaty, including charges that Moscow is violating the pact by erecting a large radar facility in Siberia.

In response to questions, McFarlane said "it is simply too soon" even to consider limitations on testing elements of the Stars Wars plan, currently in the laboratory-research phase. He also indicated that it is "premature" to place restrictions on antisatellite weapons, "which are not yet at hand on our side."

Field-testing as well as development of antimissile system components is prohibited by the ABM Treaty.

Testing and development of antisatellite weapons is not prohibited if such weapons have no antimissile purpose. The Soviets have tested a rudimentary antisatellite system; the first test of a U.S. system against a target in space has been postponed at least until June.

In the strategic arms area, McFarlane said Reagan has approved exploring trade-offs between heavy missiles, in which the Soviets have an advantage in numbers and power, and bombers and air-launched cruise missiles, in which the United States has an advantage. This concept was presented to Soviet negotiators in Geneva in October 1983, about two months before Moscow terminated that set of strategic arms negotiations.

McFarlane indicated that a new element in the U.S. position is Reagan's establishment of overall goals for reductions, with greater flexibility for the negotiators to seek this level through gradual phases. McFarlane spoke of a previous goal of a 5,000-warhead limit on each side in the life of a new treaty as "reasonable." Of six options for exploration, he said, Reagan "chose all six."

The National Security Council adviser said Reagan also has authorized positions that make possible the bridging of differences between U.S. and Soviet positions on the method of counting strategic weaponry: launchers versus warheads and missile "throw-weight."

In the intermediate-range negotiations, McFarlane reported, U.S. positions in the previous Geneva negotiations on this subject continue to provide "a satisfactory framework in which a good agreement can be reached."

He said the United States is ready to agree on "equal levels" of intermediate-range warheads with the Soviets: in return for equivalent Soviet restrictions, Washington would not necessarily deploy all 108 Pershing II missiles scheduled to be sent to Western Europe. Such a position was presented to the Soviets in Geneva in 1983.