South Korean opposition leader Kim Dae Jung yesterday thanked President Reagan for efforts to secure his freedom but called on the United States to speak out openly for human rights rather than pursue the "quiet diplomacy" that may have won his release from prison.

"Even if I couldn't come out, America should justly defend the principle," Kim said in an interview in a suburban Washington hotel a day after he was released from a heavily guarded Seoul hospital ostensibly to receive medical treatment in the United States.

"Human rights issues should not be a transaction between governments but should be supported by the voice of world consciousness and by the power of our people. I would like to see an America which defends the principle even if I were not released."

Kim, 57, became a symbol of opposition to military rule in South Korea following his kidnaping from a hotel in Tokyo in 1973 by the South Korean Central Intelligence Agency. He has spent much of the past decade under house arrest or in prison and escaped a death sentence for sedition only when it was commuted in January 1981, prior to the visit to Washington of South Korean President Chun Doo Hwan. The United States and Japan have intervened with the South Korean government on behalf of Kim.

Kim appeared tired but by no means frail as he spoke for almost an hour in the small hotel room surrounded by supporters. He puffed regularly on a pipe and measured his responses to questions carefully. His remarks were translated by his own aides and by The Washington Post.

He indicated he does not consider himself under any constraints while in the United States but avoided discussing major world issues, saying he had been free only a day and had not had an opportunity to fully study developments since he was last imprisoned in 1980.

He spoke at some length, however, about human rights, on his plans for his stay in the United States and on the life he led in prison.

"The main object of my stay is treatment of my illness and then later, if I have time, I will study, but I will attempt to go back. I don't have a plan to stay here for long," he said.

Kim is known to suffer from an ailment similar to arthritis that afflicts his legs, and he said he will seek treatment at a hospital somewhere in the eastern United States. He also has been invited to take up a fellowship at Harvard University.

Asked if he would be allowed to return to South Korea, he said, "On future events, I cannot say, but there are things I cannot control."

He also suggested that his departure from South Korea may not have been his first priority.

"I will not say that absolutely I had no choice," he said guardedly.

Kim's wife, Lee Hee Ho, who left South Korea with him as did two of his three sons, said last week that South Korean authorities were forcing him into exile against his will.

Kim attributed his departure to the Reagan administration's efforts, and the State Department already is pointing to his release as an example of how "quiet diplomacy" can work in contrast to a more outspoken approach to human rights.

The State Department yesterday also welcomed the amnesty granted to 1,200 prisoners, including a number of other dissident figures, following Kim's departure from the country.

Officials said at the very least the recent actions by the Chun government appeared designed to improve the political atmosphere when Secretary of State George P. Shultz visits South Korea Feb. 6-8.

The Reagan administration, in a distinct turnabout from U.S. policy under former president Jimmy Carter, has avoided public criticism of countries with which the United States has a close strategic relationship. Officials say, instead, that they can be more effective by raising issues such as human rights and democratic rule in behind-the-scenes diplomacy.

Critics of this approach argue that by adopting a low profile or being selective on matters of basic principle that the United States is abandoning its worldwide leadership role.

Kim made clear his views on this debate.

"I cannot reveal the source," Kim said yesterday, "but I heard that the Reagan administration pressed efforts to get me released through diplomatic channels, that there were strong demands from the Americans. I believe the fact I am released now is associated with those efforts.

"I would like to add that clearly when Ronald Reagan was elected there was a great concern over the human-rights issue. Of course, I thank them for what happened to me personally, but as far as I know it is widely perceived by the Korean people that the human-rights issue under the Reagan administration has been retreating. America should speak out openly for justice."

Speaking with fervor, Kim recalled the Vietnam years and argued that the United States would look upon itself and be looked upon differently had it supported democratic elements within the Vietnamese society.

"This is something I have long felt," Kim said. "And it's not because you failed in Vietnam."

"If America had defended from the beginning those who wanted a democracy and supported human rights and had rejected military dictatorship, if American did this from the beginning, then even if you still failed and communism took over . . . the American people would feel pride in the great sacrifice they have made and the whole world would regard America as a martyr of democracy and freedom."

Instead, Kim said, the Vietnam experience led only to frustration for Americans, a blow to American credibility in the world and "disappointment to those who support democracy."

"I, of course, thank the United States for what it did for me personally, but the principle is a different thing, it is something for which I want to appeal to the American government and the American people."

Looking back on his prison life, Kim, a Catholic, said his was "a lonely life, a life of solitude. I looked for God all the time. If there were no God, I would not be even in the shape I am today. For me, prayer came first."

Then, he said, he devoted his time to reading and to flowers.

Catholic publications, theological writers such as Teilhard de Chardin, the works of Plato and Arnold Toynbee's Study of History occupied his time. But it was the flowers that clearly were dear to him.

"Beside reading, I tended a flower bed. I extended the life of those flowers," he said with evident pride. "Compared to other prisoners, mine lived more than a month longer.

"And I lived by talking to the flowers. The dialogue with the flowers, that was the time most joyous to me."