The son of Rep. Larry McDonald (D-Ga.), who was among 269 passengers aboard a South Korean jetliner shot down last week by the Soviet Union, tried to deliver a protest letter to the Soviet Embassy yesterday. But an unidentified embassy employe refused to accept the letter and tossed it out through the embassy fence.

The exchange between Tryggvi McDonald, 22, and the Soviet Embassy employe in a dark blue shirt took place just outside the heavily shuttered embassy building on 16th Street NW, as reporters watched from across the street.

Afterward, McDonald, 22, the oldest of the congressman's five children, told reporters: "I explained who I was and said they killed my father, and I wanted an explanation . . . . He just said to get out of the embassy yard , they only take letters through the post."

McDonald, accompanied by his father's minister, the Rev. Mark Leuthold of Marietta, Ga., walked to the front door of the Soviet Embassy after visiting the White House and talking to National Security adviser William P. Clark. McDonald said he had urged that President Reagan take stronger measures against the Soviets for downing the plane, and that he was told by Clark the president would be taking "progressive action."

"He told us everything is not over yet," McDonald said.

McDonald spoke earlier in the day at a rally in Lafayette Square, which drew a crowd of about 750 persons, according to U.S. Park Police. The rally was sponsored by College Republicans and the National Conservative Political Action Committee (NCPAC).

They were joined by a separate group of about 200 placard-carrying demonstrators from the Collegiate Association for the Research of Principles (CARP), affiliated with the Unification Church of the Rev. Sun Myung Moon. That group earlier had burned an effigy of Soviet President Yuri Andropov on the sidewalk in front of the White House.

Speakers at the rally, including John T. (Terry) Dolan, chairman of the NCPAC, and Tong H. Kim, secretary-general of the Federation of Korean Associations, not only denounced the Soviets but also criticized Reagan for taking only limited steps against them.

McDonald, who is a senior at the University of Georgia, told the rally it is "ironic" that in death his father, who was chairman of the anticommunist John Birch Society, "has sparked unanimous support for the things he fought for his entire life: defense of America and opposition to communist tyranny."

He joined other speakers in urging a break in diplomatic relations with the Soviet Union, and seemed composed until the end of his speech. Then, his voice broke and he wiped tears from his eyes as he said: "My father died. He cannot come back. But if by dying it leads to the end of appeasement of the Soviet Union, then he has not died in vain."

Yesterday's rally marked the seventh day of protests in Washington against the shooting down of the plane that strayed off course and flew over Soviet territory. Jack Abramoff, national president of the College Republicans, said that anti-Soviet demonstrations were being held yesterday at many colleges around the country, including rallies at the University of Wisconsin at Madison and the University of Michigan at Ann Arbor.

At Georgetown University, there was an academic procession and mass to mourn the death of two university students and one alumnus who were aboard the plane.

At the mass, Georgetown's president, the Rev. Timothy J. Healy, urged about 200 persons in Dahlgren Chapel to set aside political "distraction," saying that "our mourning need not analyze the intellectual and moral bankruptcy of an ideology as paranoid as it is savage, nor should we show here our anger at how crudely the godless can absolve themselves of murder."

After he left the Soviet Embassy grounds with McDonald, Leuthold was less restrained than Healy.

"They're murderers. They're liars," declared Leuthold, who is pastor of the Northside Independent Methodist Church. "They have no regard for humanity. They don't believe in God."

Despite repeated calls, a Soviet Embassy spokesman could not be reached for comment yesterday.