The South Korean government of President Chun Doo Hwan announced a general amnesty today for 2,863 persons, including a reduction in the sentence of opposition leader Kim Dae Jung from life to 20 years in prison.

The amnesty was declared as part of the observances marking the first anniversary of Chun's inauguration. It is intended, in the words government spokesmen in Seoul, "to promote pan-national participation in the building of a democratic state, ensuring the well-being of all as well as enhancing an atmosphere of reconciliation."

The 57-year-old Kim was arrested in May 1980 in the military crackdown that vaulted Chun and a coterie of military colleagues to power in South Korea. Observers in Seoul said that the easing of Kim's sentence represented a step toward his ultimate release, possibly in another general amnesty later this year.

While the government gave no specific reasons for reducing Kim's sentence, diplomatic sources in Seoul said that the Chun government intended the move as a symbolic gesture designed to appeal to public sentiment in South Korea following recent steps toward easing domestic controls, including the ending of the country's 36-year-old midnight-to-4 a.m. curfew in early January.

These sources said that the government hoped to enhance its image internationally to bolster recent diplomatic overtures to North Korea aimed at reunifying the Korean Peninsula and to pave the way for the Summer Olympics scheduled to be held in Seoul in 1988.

Along with two dozen opposition party colleagues, Kim was convicted by a military court on charges of sedition for allegedly helping to touch off a tumultuous uprising at Kwangju in 1980. He was also charged with inciting campus and labor disorder and having engineered anti-Seoul protests from his exile abroad in the early 1970s.

In September 1980, Kim was sentenced to death, a decision an appeals court upheld in late January 1981. That decision was commuted to life imprisonment on the direct orders of Chun on Jan. 22 last year shortly before the South Korean leader visited Washington for talks with President Reagan.

The death penalty against Kim, one of South Korea's most celebrated political dissidents and its most prominent opposition leader, touched off strong international pressure from the United States, Western Europe and Japan. The disposition of Kim's case proved one of the thorniest issues in relations between the United States and South Korea during the Carter administration when Washington warned that Kim's execution would have serious consequences for diplomatic and economic ties.

The sentence of death against Kim also raised hackles in Japan, which threatened to stop its aid.

Kim's standing as an opposition leader was boosted in 1971, when he lost that year's presidential election by a slim margin to Park Chung Hee, who was assassinated in October 1979. During the rest of the 1970s, Kim spent most of his time in exile, in prison in South Korea or under house arrest in Seoul.

An outspoken critic of the Park government, Kim was freed in early 1980 in the general amnesty that followed Park's assassination and quickly declared his intention to run for president. He was considered the opposition's leading candidate before he was arrested in May 1980.

It is widely believed that Seoul is also concerned with mending delicate diplomatic ties with Japan in light of South Korea's current request for $6 billion in Japanese aid. Kim's abduction from a Tokyo hotel room in 1973, allegedly by South Korean agents, touched off a stormy quarrel with Japan.

Under the general amnesty announced today, two of Kim's colleagues in the political opposition, Ye Chun Ho and Kim Chong Wan, will be granted conditional release from prision while others convicted in connection with the 1980 uprising will have their jail terms reduced.

Meanwhile, leading Christian dissident Moon Ik Hwan will have his 10-year prison term commuted to five years. Kim Kae Won, a prominent retired Army general who was involved in the Park assassination, will have his sentence reduced from 20 years to 10 years.