In a full-scale reversal of his predecessor's policies. President Reagan yesterday rejected further withdrawal of U.S. military forces from South Korea and announced strong support for its new leader without any qualification based on human rights.

With Korean President Chun Doo Hwan at his side in a highly symbolic ceremony, Reagan declared that "the United States will remain a reliable Pacific partner and we shall maintain the strength of our forces in the Pacific area."

In a joint communique issued an hour later, Reagan announced that the United States will "resume immediately" the full range of governmental consultations with South Korea, including the security-related meetings the Carter administration had withheld in order to pressure Korea's military leadership on human rights and demoncratic procedures.

"What happens internally is an internal affair of the Republic of Korea," declared the U.S. spokesman who briefed reporters at the White House. The Reagan administration, according to the State Department, went to the unusual length of asking Congress to postpone publication of a report critical of human rights conditions in Korea, written in the last days of the Carter administration, in order not to embarrass the high-level visitor.

Chun, a former general who seized power in a midnight coup a little more than a year ago, said he was "happy" at the "firm assurances" given by Reagan. The upshot of the day's events was to bestow full legitimacy and approval on Chun for the first time in the eyes of official Washington, and to fully normalize U.S. relations with Chun's regime.

The Korean leader is expected to be elected to a full seven-year term as president in indirect balloting later this month. Any lingering doubt of his election against token opposition, with most of his potential competitors banned from public life, was eradicated by the highly visible and unqualified endorsement he received here yesterday.

Chun's share in the rapprochement was his action, shortly after announcement of the official visit here, commuting the death sentence of Kim Dae Jung, South Korea's best-known opposition political figure. The incoming Reagan administration had joined the Carter administration in urging clemency for Kim. Though "a deal" for Kim's life has been denied, Reagan administration sources said it was made clear that there would be no trip to Washington if Kim were executed.

A group of 52 U.S. Protestant church leaders, in a statement released yesterday, urged Reagan to press Chun to "restore freedom of opinion and expression" to South Korea and to free Kim and other imprisoned political figures. The religious leaders were led by Dr. Claire Randall, general secretary of the National Council of Churches, and William P. Thompson, stated clerk of the United Presbyterian Church in the U.S.A.

The two presidents, however, did not discuss Kim, according to the U.S. briefing, and talked about human rights only "in general terms." The spokesman said the Reagan administration is "not looking backward" at the past but training its eyes forward toward a more cooperative future relationship.

Although White House officials had indicated that the briefing about the Reagan-Chun meeting was to be conducted by Richard V. Allen, the presidential assistant for national security, he was not in evidence at the session. Instead, reporters were briefed by a senior State Department official with responsibilities in Asia, a further sign of the public and policy dominance of Foggy Bottom in the reign of Secretary of State Alexander M. Haig Jr. The official cannot be identified, under the rules of the briefing.

The twin aims of withdrawing U.S. ground troops and advancing human rights were central and controversial elements of the Korea policy of the Carter administration. Because of strong opposition in Congress and abroad, President Carter was forced to shelve his gradual troop withdrawal program in July 1979, though he never formally gave up the aim.

Regarding human rights, the Democratic administration was critical to the end of the political repression that accompanied Chun's seizure of power. One sign of this was the Carter administration's refusal to schedule the annual "Security Consultative Meeting" involving the defense ministers of the two countries. Then-Secretary of Defense Harold Brown visited Seoul last December, but only briefly, and with the objective of making the U.S. case for commutation of the death sentence against Kim.

As explained to reporters at the White House, the new U.S. policy will depart from the "rather rocky road" of the past administration by placing security as unchallenged first priority in Washington-Seoul relations. To this end, the joint communique promised that the United States will make available "appropriate weapons systems and defense industry technology." No details were given.

"It will not be the policy of this administration to withdraw any forces from Korea," Reagan was officially reported as having told Chun in their meeting.

At the end of his first meeting with an Asian leader, Reagan went out of his way to reassure not only Korea but Japan, Australia, New Zealand and the Association of South East Asian Nations (asean), mentioning all of them by name, that U.S. alliance and support in Asia will be on a par with that in Europe.