President Reagan yesterday accused the Soviet Union of "flagrantly" lying about the downing of a South Korean airliner with 269 crew members and passengers, including at least 52 Americans, and questioned whether the United States can continue to talk "with a state whose values permit such atrocities."

In the strongest denunciation he has delivered of Soviet behavior as president, Reagan suggested that the Soviets had gone beyond "certain irreducible standards of civilized behavior" and had violated the "tradition in the civilized world" of helping pilots who are lost or in distress.

Standing with his wife, Nancy, on the field of the Point Mugu Naval Air Station on the California coast before returning to Washington to meet with National Security Council, Reagan read solemnly from a prepared text:

"What can be said about Soviet credibility when they so flagrantly lie about such a heinous act? What can be the scope of legitimate mutual discourse with a state whose values permit such atrocities, and what are we to make of a regime which establishes one set of standards for itself and another for the rest of humankind?"

Presidential spokesman Larry Speakes told reporters aboard Air Force One en route to Washington that the president had received a list of options for responding to the Soviet action, which was to be discussed at the White House last night.

A number of U.S. officials said yesterday, however, that they believe that Reagan will find it difficult to go much beyond rhetorical retaliation and such relatively limited sanctions as seeking international restrictions on Soviet air traffic, inhibiting trade in areas other than grain sales, placing new restrictions on Soviet diplomatic personnel and putting off tentative plans for talks on a new scientific and cultural exchange agreement and the opening of consulates in New York and Kiev.

A senior administration official traveling with Reagan said, for example, "I would not look for us to discontinue our discussions with the Soviets on nuclear arms control because the stakes are too high. We would not be serving our own country or the world at large should we stop our efforts to achieve true arms reduction."

Speakes said, "Arms control is a very important issue, probably one of the major foreign policy emphasis of our administration."

State Department spokesman John Hughes added that he was unaware of any plans to call off or postpone next week's scheduled resumption of the U.S.-Soviet negotiations on reducing medium-range nuclear missiles in Europe.

Secretary of State George P. Shultz said Thursday that he still intends to meet next week with Soviet Foreign Minister Andrei A. Gromyko in Madrid, and Hughes said yesterday, "So far as I know, there has been no change of that position."

Administration sources also said it would be difficult for Reagan to cancel the new, five-year agreement to sell American grain to the Soviets.

Canceling the grain sales agreement would risk an outcry from American farmers and their political representatives. It also would contradict Reagan's past opposition to the grain embargo imposed by President Carter against the Soviets after their invasion of Afghanistan. And it would violate guarantees in the new agreement that make it legally difficult to impose a new embargo.

In his statement yesterday, Reagan portrayed the Soviet regime as having gone beyond the standards of civilized behavior accepted by the rest of the world. His remarks appeared to go further than expressing outrage about the incident, to suggesting that the United States might take some unspecified actions against Moscow.

In rhetoric and practice, Reagan has been both harsh and conciliatory toward the Soviets during his first 2 1/2 years in office. He decried Soviet-backed repression of human rights in Poland and the invasion of Afghanistan. But he also cleared the way eventually for expanded grain sales to the Soviets, and recently approved the sale of pipe-laying equipment built by Caterpillar Tractor Co. for use in building the Soviet natural gas pipeline in Europe.

Reagan has always expressed deep suspicions about Soviet behavior and intentions, however. He has accused them of fomenting revolution in Central America; he has predicted that communism is in its final throes, and earlier this year he described the Soviet Union as an "evil empire" in a speech to a group of evangelicals in Orlando.

It was clear from his statement yesterday that the Soviet attack on Korean Air Lines Flight 007 had reinforced Reagan's basic convictions about the Soviet system: that it lacks the basic moral standards of the West.

He described the shooting down of the unarmed jetliner as a "barbaric act." He repeatedly sought to contrast the Soviet action with accepted behavior in the "civilized" world.

"Our first emotions are anger, disbelief and profound sadness," Reagan said. "While events in Afghanistan and elsewhere have left few illusions about the willingness of the Soviet Union to advance its interests through violence and intimidation, all of us had hoped that certain irreducible standards of civilized behavior nonetheless obtained.

"But this event shocks the sensibilities of people everywhere. The tradition in the civilized world has always been to offer help to mariners and pilots who are lost or in distress on the sea or in the air," Reagan said. "Where human life is valued, extraordinary efforts are extended to preserve and protect it . . . . "

He added that, "It's essential that as civilized societies we ask searching questions about the nature of regimes where such standards do not apply.

"Beyond these emotions," he went on, "the world notes the stark contrast that exists between Soviet words and deeds.

"What can we think of a regime that so broadly trumpets its vision of peace and global disarmament and yet so callously and quickly commits a terrorist act that sacrifices the lives of innocent human beings?"

Reagan was accompanied on his return to Washington by White House counselor Edwin Meese III, national security affairs adviser William P. Clark and deputy White House chief of staff Michael K. Deaver. The White House said Reagan will not make a Labor Day speech in Florida on Monday as planned. Vice President Bush will make the appearance instead.

"We have not as yet received any further statement from the Soviet government," Speakes said. "We are working very closely with our allies, particularly South Korea and Japan."

Reagan was briefed yesterday morning on international reaction to the incident as he flew by helicopter from his ranch. Speakes said Reagan was told that "it is the strongest international reaction to any incident in recent memory" and came from South Korea, Britain, Italy, Australia, France, Brazil, Canada and Japan.

The dilemma facing Reagan in deciding how to respond to the Soviets was underscored Thursday night by Lawrence S. Eagleburger, undersecretary of state for political affairs and the administration's top expert on East-West relations. He said, "An act like this cannot help but have an impact on our relationship and on our attitude toward the Soviet Union."

But he also warned, "We are going to have to deal with and live on the same planet with the Soviet Union."

Eagleburger was reflecting the view of State Department experts, who say they believe a complete break with the Soviets would be counterproductive. That attitude seems certain to encounter opposition from staunchly anti-communist conservatives who form the core of Reagan's political support and from hard-line White House advisers sympathetic to their attitudes.

However, while they can bring considerable pressure to bear on Reagan, the president also must take into account counter pressures from other domestic interest groups, such as farmers, and from U.S. allies.

The NATO allies are deeply concerned that any sign of flagging U.S. interest in reaching arms-control agreements with Moscow will give new impetus to the anti-nuclear movement in western Europe and create new obstacles to the scheduled deployment there later this year of new U.S. medium-range nuclear missiles.

Shultz's decision to go ahead with the Madrid trip also was prompted by a desire to avoid an irreparable breach with Moscow and to reassure the allies that Washington is sensitive to their concerns.

The ostensible purpose of the trip is to mark the conclusion of the Madrid Conference on European Security and Cooperation, which is regarded as highly important throughout western Europe.

However, U.S. officials said yesterday that Shultz plans to use the Madrid gathering of foreign ministers as a forum for focusing attention on the plane incident and for seeking broad-based, European condemnation of the Soviets.

"We think the format of discussion and the priority of the issues might change," Hughes said. "This particular incident looms large and first on the agenda."

In the midst of the furor over the reported shooting down of the South Korean plane, the Agriculture Department announced yesterday that the Soviets had purchased at undisclosed prices 750,000 tons of grain and 190,000 tons of soybeans. That followed an announcement Thursday that the Soviets were buying 900,000 tons of wheat and corn.

The size of these transactions, and their economic impact for American farmers hard hit by recession and drought, was cited by many administration officials as the main reason why Reagan is not expected to heed the call of Senate Minority Leader Robert C. Byrd (D-W.Va.) and others for canceling the new grain agreement.

Byrd's proposal immediately was opposed by members of the House and Senate Agriculture Committees and by other farm-state legislators who said cancellation would hurt American farmers and be ineffective because the Soviet Union would only turn to other countries for its grain purchases as it did in the last embargo. Agriculture Secretary John R. Block said, "Grain embargoes have always been losers for farmers. I don't think they make any sense."