Congressional leaders reacted angrily yesterday to a report that the Soviet Union downed a South Korean airliner, calling it murderous and barbaric, but disagreeing about what should be done.

Lawmakers suggested retaliatory steps ranging from canceling the new grain pact with Moscow to calling for a U.N. Security Council meeting and pressing other nations to suspend all commercial airline flights into the Soviet Union.

Some saw the downing of the plane as threatening to worsen relations between the superpowers, which have only recently begun to warm slightly.

"Attacking an unarmed civilian plane is like attacking a school bus," Rep. Thomas F. Hartnett (R-S.C.) said.

"What secret is so sensitive or what soil is so private to guard it over 269 innocent people who were condemned to fiery death without hearing, without appeal and without mercy," said Sen. Charles McC. Mathias Jr. (R-Md.).

But despite the "reprehensible" incident, the United States still has "an obligation to continue the dialogue with the Soviet Union in the quest for peace," said Senate Majority Leader Howard H. Baker Jr. (R-Tenn.). "I don't think there is anything else that can or should be done at this time," he said.

The government should "keep cool and keep searching for all the facts before taking any action," said Rep. Thomas A. Luken (D-Ohio).

"The whole matter could be attributable to a single, trigger-happy Soviet pilot," said Rep. George W. Gekas (R-Pa.).

"It's very important we determine the degree of involvement of the Soviet Union ," said Rep. James J. Florio (D-N.J.).

House Majority Leader James C. Wright Jr. (D-Tex.) said the incident "profoundly affects the future of our relationship with the Soviet Union." But he added: "We need to wait until we assemble all the information. We need to approach this with clear eyes and cool heads and know exactly what happened before we respond."

"It doesn't matter whether the shooting was a matter of ill-trained pilots or misguided Soviet policy stemming from their inferiority complexes about the free world," said Sen. Walter D. Huddleston (D-Ky.), a member of the defense subcommittee of the Senate Appropriations Committee. "There is no excuse for the slaughter of hundreds of human beings."

"Explanations and apologies will not suffice," said House Speaker Thomas P. O'Neill Jr. (D-Mass.). "Only direct action by the leadership of the Soviet Union to prevent a recurrence of this tragedy will do."

Senate Minority Leader Robert C. Byrd (D-W.Va.) said he believed the United States should cancel its recently negotiated grain-sale agreement with the Soviet Union.

Sen. Edward M. Kennedy (D-Mass.) urged western nations to suspend all commercial airline flights into the Soviet Union until Moscow gives a full accounting of its "cowardly" attack on the airliner.

House Minority Leader Robert H. Michel (R-Ill.) and Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) also called for an immediate session of the Security Council.

Rep. Henry J. Hyde (R-Ill.), a member of the House Foreign Affairs Committee, said that unless the Soviets offer a quick apology and restitution for the action the United States should "let a chill set in" in relations with Moscow.

Sen. Patrick J. Leahy (D-Vt.) said that because of the attacks on U.S. Marines in Lebanon and the airliner incident, the chairmen of the Senate Armed Services, Foreign Relations and Intelligence committees should return to Washington and call meetings of their panels for briefings.

Sen. Howard M. Metzenbaum (D-Ohio), who returned Wednesday night from the Soviet Union with other members of a Senate delegation, said proof that the Soviets had downed the plane "would have a very negative impact as far as the arms control negotiations are concerned, and our total relationship with the Soviets."

Rep. Hal Daub (R-Neb.) cancelled his trip to visit Soviet dissidents in Moscow as a guest of the Omaha branch of the Union Council on Soviet Jewry.

Former president Jimmy Carter called the incident "inexcusable. . . . There is no provocation imaginable that could warrant such an action."

Former secretary of state Henry A. Kissinger said the shooting was an "outrageous, unforgivable act" that would require a "great deal of explanation."

Former vice president Walter F. Mondale, an announced candidate for the 1984 Democratic presidential nomination, said he joined with others in "demanding a full explanation from the Soviet government for this murderous deed. I believe the United States should take this matter immediately to the U.N. Security Council."

Sen. Alan Cranston (D-Calif.), another announced presidential candidate, called the attack "tragic and alarming," and said it highlights "the trigger-happy dangers in which we all now live and the incredible tensions with the Soviet Union."