Amid widespread expressions of American outrage and revulsion at the shooting down by a Soviet jet fighter of a South Korean airliner with 269 aboard, President Reagan yesterday demanded "an immediate and full" explanation from Moscow "for this appalling and wanton misdeed."

Reagan also decided to cut short his California vacation and return to Washington today to meet with his top advisers and congressional leaders over the weekend about the shooting down of the airliner, in which Rep. Larry McDonald (D-Ga.) and at least 30 other Americans died.

The United States and South Korea also called yesterday for an urgent meeting of the U.N. Security Council today to hear their charges about the Soviet action. U.S. officials sought to stress the international implications of the crisis, as well as its potential impact on U.S.-Soviet relations.

As congressional leaders and other political figures reacted with strong statements, Senate Minority Leader Robert C. Byrd (D-W.Va.) called for cancellation of a new $10 billion grain sale agreement just signed with Moscow. The first sale of American wheat and corn to the Soviets under that agreement was announced, coincidentally, yesterday.

At the State Department, a grim-faced Secretary of State George P. Shultz told reporters, "We can see no excuse whatsoever for this appalling act." U.S. officials were stunned by the prospect that senior Soviet authorities would authorize the shooting down of a clearly marked commercial airliner that had strayed off course.

U.S. sources, relying on the ability of American and possibly Japanese electronic listening posts to eavesdrop on Soviet military communications, said that communications between the Soviet fighter pilots and their ground commanders also extended back to higher authorities in Moscow.

They said this means that high-level Soviet authorities were informed about the presence of the Korean jetliner as it strayed in and out of Soviet airspace on two occasions during a 2 1/2-hour period in which it was being tracked by Soviet radars. The officials said that they did not know how high-ranking the Moscow officials were or whether they were military or civilian.

Presidential spokesman Larry Speakes also talked of the "barbarity" of the Soviet "government" in shooting down the plane.

There was some uncertainty evident here yesterday, however, about whether an order to fire was given to the Soviet pilot from ground commanders.

One source said there definitely was such an order. At the State Department, Assistant Secretary Richard Burt declined to give direct answers to questions on this point, but stressed that the Soviet pilot was under constant ground control and "described and discussed a sequence of movements and actions he was taking to engage this aircraft, including the arming and firing of a missile."

Other administration officials said that they are not entirely sure that an order to fire was given to the Soviet pilot from the ground. But they said "it is hard to imagine" that the pilot, who was under tight control from the ground at all times, could have gotten nervous and fired without such an order. Burt also noted that the Soviets traditionally exercise extremely tight control over their military forces.

A Japanese news agency claimed that a transcript of the Soviet air-to-ground communications taped by Japanese intelligence reports a direct order to fire. But State Department spokesman John Hughes said late yesterday that there was no clear-cut transmission like that in the U.S. record of the conversations.

Shultz, who provided a detailed, minute-by-minute account of the attack to reporters earlier yesterday, left out of his account any direct reference to an order to fire.

Officials said the Soviet jet fighters were probably MiG23s, but there were some reports that identified them as older Su15s.

Speakes, briefing reporters in California where Reagan is vacationing, said the United States was weighing options it might have for retaliation against the Soviets. But Speakes and other officials declined to speculate on what those might be.

"It depends on the Soviet explanation of it," Speakes said. "We'll await a Soviet reaction. The Soviet Union owes an explanation to the world about how and why this tragedy occurred."

The State Department said later that Shultz had received a message from Soviet Foreign Minister Andrei A. Gromyko but that the Reagan administration found it "totally inadequate."

Gromyko's response, officials said, was an almost verbatim copy of a Tass news agency report earlier in the day. It did not admit that the Soviets shot down the plane, they said.

The Tass account also claimed that the Korean airliner did not have navigation lights. Yet, sources here, also apparently relying on intercepted communications, said the Soviet pilot told ground stations that the jetliner was blinking its lights.

The shooting down of Korean Air Lines Flight 007 comes at what had, until now, been viewed as a period in which U.S.-Soviet relations might be undergoing a slight change for the better. While no breakthroughs have been made in the key issues separating the two countries, there has been some slight progress on nuclear arms control issues lately and an expansion of U.S.-Soviet trade as Reagan recently approved the grain deal and the sale of pipe-laying equipment to the Soviet Union.

Shultz also is scheduled to leave Washington next Tuesday for Madrid to mark the ending of a 35-nation review conference on European security, which had also been seen as a sign of narrowing U.S.-Soviet differences. Shultz said yesterday that he will go through with his scheduled meeting with Gromyko in Madrid, but other officials said it was highly likely that any additional business Washington was prepared to do with Moscow will be postponed.

The Reagan administration's caution in describing retaliatory measures it might take also was reflected by Senate Majority Leader Howard H. Baker Jr. (R-Tenn.). Despite the "reprehensible" incident, Baker said, the United States still has "an obligation to continue the dialogue with the Soviet Union in the quest for peace."

Sen. John Glenn (D-Ohio), campaigning in Iowa for the Democratic presidential nomination, said he deplored the Soviet act, but also urged that "any reaction by our government . . . await further determination of all the facts."

Leaders of a coalition of conservative groups, however, outlined a list of immediate demands that included: canceling the grain deal and a recently approved sale of American pipeline equipment to the Soviet Union; suspending arms control negotiations with the Soviets; recalling the U.S. ambassador to the Soviet Union; denying Soviet ships access to American ports; terminating all trade with the Soviet Union; ending all cultural and scientific exchanges, and expelling all Soviet diplomatic, trade and cultural personnel in the United States.

Rep. Philip M. Crane (R-Ill.) called for an "economic embargo" against the Soviets.

Former vice president Walter F. Mondale, also a Democratic presidential contender, called the Soviet action "barbarous and despicable" and urged U.N. action.

Among the harshest congressional reactions, Sen. Alfonse M. D'Amato (R-N.Y.) called the incident "nothing less than murder," and Rep. William L. Dickinson (R-Ala.) said it was "first-degree, cold-blooded murder."

The magnitude of the incident and the apparent necessity for an iron-clad American charge that the Soviets had indeed shot the plane down and knew what they were doing, produced an unprecedented glimpse into American electronic eavesdropping capabilities.

In making his statement to reporters yesterday morning, Shultz provided the exact time that the Soviet fighter pilot "reported visual contact" with the 747 jetliner. Nine minutes later, the Soviet pilot told his ground controllers he was 10,000 meters from the KAL plane. Five minutes later, Shultz said, the pilot reported he had "fired a missile and the target was destroyed."

The United States maintains extensive electronic listening posts around the world, operated by the super-secret National Security Agency. Some of the most sensitive outposts are in Japan, which keeps an electronic ear especially on Sakhalin Island, a Soviet outpost just 100 miles north of Japan that the Soviets have turned into a heavily fortified forward air base.

The area into which the South Korean jetliner strayed is among the most sensitive in the Soviet Union. Aside from Sakhalin Island and its fortifications, the plane also overflew, according to Shultz, the Kamchatka Peninsula where Soviet missile test firings take place, and the Sea of Okhotsk, which is a key maneuvering area for Soviet Pacific fleet submarines.

Despite the communications monitoring capability, however, officials made clear that they did not know what had happened to the Korean jet for several hours after it had been shot down. They explained that, in part, it takes an extensive search of recordings made by surveillance radars to first find the data, then it took more time to translate it from Russian into Japanese and then English.

The jetliner, according to State Department officials, actually was shot down as it was leaving Soviet airspace and crashed at a point roughly 30 miles west of the southernmost tip of Sakhalin.

Speakes said Reagan was first briefed about the plane at 7:30 p.m. California time (10:30 p.m. EDT) Wednesday, about the same time that Burt said the State Department went on alert and formed a special operations group with officials from the Pentagon, White House and intelligence agencies.

Burt said that as it became increasingly apparent that something tragic had happened, he telephoned Soviet charge d'affaires Oleg M. Sokolov, waking him up in the middle of the night, to ask for an explanation. Calls also were placed to the U.S. Embassy in Moscow with instructions to inquire immediately at the Soviet Foreign Ministry. Burt said the Soviets provided no information.

By early yesterday, Burt said, it was clear what had happened and Sokolov was called in to the State Department for an explanation but still had none to give.