Kim Dae Jung, South Korea's principal opposition leader, was sentenced to death this morning for allegedly plotting a rebellion and forming an antistate organization.

A military tribunal handed down the sentence after finding Kim guilty at the close of a trail that lasted more than a month.

Unless the verdict is overturned on appeal, the 54-year-old political leader will be hanged. His life also could be spared by the country's new president, Chun Doo Hwan.

[There was no immediate comment from U.S. officials in Washington, who have expressed great confidence in recent days that Kim will not be executed even if sentenced to die. Their confidence is evidently based on explicit assuances from the new leadership in Seoul.]

[Although some officials had hoped that a death sentence would not be imposed, the consensus of those dealing with South Korean affairs was that the official scenario in Seoul called for President Chum or the highest Korean court to be in position to commute Kim's sentence as a gesture and symbol of mercy.]

The military court gave its decision about 10 minutes after convening in a military headquarters trial court and began pasing sentences ranging from two to 20 years on 23 other defendants accused of conspiring with Kim.

Kim has been the country's most prominent political opposition leader for years and narrowly lost the presidential election in 1971 to the late Park Chung Hee.

He was arrested in a military crackdown on May 17 as Chun came to power by pushing aside the civilian government.

He was accused of planning an insurrection last May, when thousands of students were demonstrating against martial law and demanding a return to free, democratic elections. He also was charged with forming an antistate organization in the early 1970s while he was in exile in Japan.

During his trial, Kim said he had been held in an isolated cell for 60 days, subjected to persistent questioning and brought to the brink of torture.

He denied plotting to overthrow the govenment and said he had never formed an antistate organizatiion in Japan, pointing out he had been kidnaped from Tokyo by government agents before the organization in question was formed.

He had acknowledged violating some martial-law edicts last spring and admitted receiving money in violation of the foreign exchange act. Neither of those charges carries a death penalty.

The verdict could be set aside by the current martial-law commander, Gen. Lee Hui Sung, or by the Supreme Court. If both of those appeals fail, his fate would be in the hands of Chun, who must sign an order for his execution.

Among the other defendants was a prominent American-educated pastor, the Rev. Moon Ik Hwan, 62, and several students and professors.

In a dramatic final statement to the military court last week, Kim denied that he ever intended to seize power through an insurrection and said his speeches and activities last spring were intended only to advance a legitimate presidential campaign.

He also denied having communist connections and asked the court to recall that he had been given a pardon for past crimes by the government.

"I have made every effort to achieve democracy, but I never tried to seize power by an insurrection," Kim told the court. Trials such as his, he added, "should never happen in this land again."

Kim's wife said as the trial began late in August that the family had been unable to find lawyers of its own choosing. the court later appointed a defense counsel who had once been a military prosecutor.

Several of the defendants declared during their trial that they had been beaten and tortured by military investigators who forced them to make false statements, several of which incriminated Kim. Most of them recanted those confessions when they were given an opportunity to do during the trial.

The United States is reported to have warned the Chun government that a death penalty for Kim would seriously hamper relations between the two countries, and Japan has delivered similar expressions of concern. A State Department spokesman has said the charges against Kim appeared to be "far-fetched" and mostly involved legitimate election campaigning.

A State Department legal adviser has been present during most of the military tribunal hearings here.

Kim is regarded as South Korea's most eloquent opposition leader and had spent much of his life since 1971 criticizing the government of the late president Park, who was assassinated last October.

Kim was a strong critic of the interim government that took over after Park's death, contending that it was not being specific about promises to hold free elections.

Shortly before he was arrested on May 17, Kim was making plans to form a new opposition party and run for president as soon as elections were scheduled. He begun extensive speaking campaigns and was forming a staff to run his election offices when the military cracked down.