They gathered at Constitution Hall yesterday to memorialize Rep. Larry P. McDonald and the 268 other victims of Korean Air Lines Flight 007, but the crowd of 3,700 was soon cheering as successive speakers condemned the Soviet Union as a nation that can never be trusted.

Some at the service were carrying Western Union mailgram invitations sent by the sponsoring Conservative Caucus, a McLean research organization. McDonald's wife, Kathryn, his children and mother were present, along with several South Korean Embassy officials and such prominent conservatives as Sen. Jesse Helms (R-N.C.), the Rev. Jerry Falwell and former Georgia Gov. Lester Maddox.

They came to hear the five-term Democratic congressman championed as "the leading opponent of Communism in Congress." The Soviet Union's action in shooting down an unarmed passenger plane was akin to the Japanese attack on Pearl Harbor, Falwell told the crowd.

Some, like Cheryl Amisial of Silver Spring--who said her first-grade teacher was on Flight 007--were there simply to mourn lost friends and relatives. But others said the banners "Victory over Communism" and "We will not forget" spoke for them.

"I think this was absolutely fantastic," said Arlington resident Flora Schroebel. "I hope Mr. Reagan gets the message. We have to take stronger action against the Russians."

The crowd was somber and reserved through selections from the U.S. Navy Band and as McDonald's eldest son, Tryggvi, said the Pledge of Allegiance. Although most speakers stopped short of offering concrete alternatives, Howard Phillips, chairman of the Conservative Caucus, drew the first applause as he criticized the extent of the U.S. response to the downing of the plane.

"In the absence of decisive, rapid, retributive action a dangerous message is communicated: paralysis, confusion, impotence and fear," said Phillips to strong applause. The world "will not be enhanced by bringing atheistic murderers into the community of civilized nations. . . . It is not vengeance we seek but simple justice and godly retribution."

South Korean Ambassador Byong Hion Lew led a large contingent that included a member of the South Korean Supreme Court, Yong Chul Kim, and the Korean Church Choirs of Washington. South Korean President Chung Doo Hwan sent a huge floral arrangement. McDonald, who also served as national chairman of the John Birch Society, had been flying to South Korea for ceremonies honoring the defense agreement between the United States and that country.

"This is an outrage against everything that civilizations hold dear," Lew said. "We must resolve that the departed have not died in vain." The theme that much must be learned from the incident and that it must be used to awaken a complacent United States to support a tougher stance against the Soviets was followed up by others.

"We sorrow, but we sorrow not as others who have no hope," said Falwell, who earlier in the day had conducted a service for McDonald at the Thomas Road Baptist Church in Lynchburg, Va. "Dec. 7th, 1941, and Aug. 31st, 1983, were days of infamy, but on the positive side, they will be remembered as days that helped to galvanize, nationally and internationally."

"This needs to be supported," said Yugoslavian native Becir Meamedovic, who now lives in Washington. "The most important thing is to show our stand against communism, whatever political party we belong to. We have to stand in defense of our nation and our freedom."

John Rees is the Washington bureau chief for a weekly news magazine, but he wasn't covering the service yesterday. He came instead, he said, "to show respect for the memory of Congressman McDonald and what he stood for. We have to show an awareness of the precise nature of Soviet aggression."