KWANGJU, SOUTH KOREA, SEPT. 8 -- A huge and peaceful crowd filled the streets of this provincial capital today to welcome home South Korea's best-known opposition leader, Kim Dae Jung, after an absence of 15 years.

Kim returned to his native province to reopen his political career after years of imprisonment, exile and house arrest in Seoul. Hundreds of thousands of well-wishers lined the streets, peered down from rooftops and hung out of windows, dramatically refuting those who had suggested that Kim was a faded politician with no popular following or an incendiary who would incite violence wherever he spoke.

Kim, 63, steadfastly denied that the visit was the campaign rally it appeared to be, and he canceled earlier plans to visit cities outside his home province where his popularity might have been more rigorously tested. But his ability to generate and control such a good-natured throng demonstrated that, despite years of vilification and ostracism by the officially controlled press, Kim remains a forceful presence in Korean politics.

"Every citizen had been harboring deep inside a desire to see him, and now it is exploding out all at once," said Im Kyung Min, 29, a white-collar worker. "So we all had to come out to welcome him."

Kim won 46 percent of the vote in the 1971 presidential election, South Korea's last genuinely contested vote. Since then, he has been kidnaped from a Tokyo hotel and nearly killed by South Korean security agents, jailed, put under house arrest, released, jailed and sentenced to death, sent into exile and put under house arrest again.

As part of a series of government concessions in response to widespread street protests in June, Kim was freed from house arrest and given permission to become a public figure again. But some military officers and conservatives in the ruling camp still regard him as radical and demagogic.

The government has said it will allow direct presidential elections in December. Either Kim or the nation's other chief opposition leader, Kim Young Sam, is expected to run against ruling party Chairman Roh Tae Woo.

Both Kims have said that they will agree on one candidate so as not to split the opposition vote, and neither has officially declared. But Kim Dae Jung has acted sufficiently like a candidate to prompt some generals to begin a whispering campaign against him, suggesting that his nomination could lead to another military coup.

The ruling party also seemed eager to distract attention from Kim's trip today when it announced Roh's impending visit to the United States. Officials said Roh will meet with President Reagan, which Kim said today would be "somewhat unusual . . . because he is not a government leader."

As Kim has throughout the summer, he appeared eager to demonstrate his popularity while stressing that he does not want to be divisive. Disavowing any political purpose in his trip, he said he wanted only to listen to people from whom he has long been kept apart and to thank them for their support. His last words tonight were a plea for everyone to return home peacefully.

"I want to be careful not to create any disturbance, not to give a good excuse to this government," Kim said in an interview before reaching Kwangju this morning. "We must be on the alert to prevent military involvement in politics."

But his journey to Kwangju was hardly the low-key affair he had described. During the day Kim visited a cathedral to meet with Catholic Archbishop Yoon Kong Hee and met with about 200 Protestant leaders. The crowds grew larger, station by station, as Kim rode the Saemaul Express train from Seoul to Kwangju, and his arrival ceremony was complete with greetings from a little girl in a wheelchair, banners welcoming "Our Nation's Hope," and tens of thousands of wildly cheering supporters.

As the day progressed, the crowds grew steadily larger. They tagged along Kim's parade route any way they could, doubling up on motorbikes or piling into the back of open trucks. They remained friendly despite suffocating closeness at times, offering each other soft drinks and waving cheerfully to every politician in Kim's entourage.

Supporters ranged from elderly men in white suits, straw hats and wispy goatees to young people who could not possibly remember the 1971 election, when Kim last received extensive press coverage. in the press. One employe of an electronics store wearing an Arnold Palmer sports shirt said little of his knowledge of Kim came from newspapers or television.

"We younger men have always heard about him from older people," said Park Sung Hung, 29. "We believe he should be a candidate, and when he runs, we will support him."

By evening, as Kim and his wife drove slowly through the city, waving to the crowds from the back of an open truck and wearing garlands of flowers, the crowd appeared to be more than 200,000. The parade ended in the central plaza, where more than 200 civilians died in an antigovernment uprising in 1980.

At about 9 p.m., the crowd peacefully dispersed, with no riot police in sight. The quiet end to the day was almost unheard-of in South Korean politics, where at least a minority of crowds usually stays confront police -- who generally gather in large and visible numbers.

"In all my life, I've never seen such a large crowd be totally peaceful," said Yang Soon Jik, vice president of the opposition Reunification Democratic party and a supporter of Kim.