SEOUL, NOV. 29 -- South Korean opposition leader Kim Dae Jung successfully staged the biggest rally of his political career here today, while thousands of his supporters booed and threw rocks at ruling party candidate Roh Tae Woo in the antigovernment bastion of Kwangju.

The two events demonstrated, in different ways, the troubles that South Korea may face after its Dec. 16 presidential election, the first democratic contest in 16 years. Kim suggested that a "serious uprising" may ensue if he loses, while Roh's bitter reception showed how divided this nation of 42 million has become.

More than a million people turned out on a sunny but below-freezing afternoon in Seoul to show their support for Kim, who is running for president after years of persecution by military-installed regimes. Kim said the massive turnout proves he will win unless President Chun Doo Hwan's government commits election fraud.

"As of today, Kim Dae Jung has won the election," Kim told the roaring crowd. Then, in one of the strongest statements of his campaign, Kim suggested that Chun may meet the fate of two of his predecessors, one of whom was forced into exile by a student uprising in 1960 and the other of whom was assassinated in 1979.

"If the present government now blocks a free and fair election, there will be a second Syngman Rhee or a second Park Chung Hee," he said.

Meanwhile, Chun's favored successor, former general Roh, ventured into Kim's home province and addressed a crowd of about 50,000, almost all of them Kim supporters. The crowd burned Roh leaflets, stomped on Roh photographs and threw stones, eggs and sticks while Roh delivered an eight-minute speech surrounded by aides armed with Plexiglas shields.

Tear gas drifted across the raucous rally, mixing with smoke from bonfires of campaign posters, but it was unclear whether the gas grenades were hurled by protesters or some of the thousands of riot policemen deployed to guard Roh.

Roh was not hurt, but several aides and reporters were injured when they were struck by objects.

Kwangju is not only the capital of Kim's home province but the site of a bloody 1980 clash between troops and unarmed civilians protesting the coup that brought Chun and Roh to power.

Even Kim's opposition rival, Kim Young Sam, met a hostile reception when he recently tried to campaign in Kwangju, so Roh's troubles today did not come as a surprise. But political observers said Roh had to demonstrate that he was not afraid to visit any provincial capital, and he took comfort in the fact that -- unlike Kim Young Sam, who scurried off the podium after two minutes -- he could ignore the incoming missiles long enough to finish his brief address.

"I feel it's fortunate that I finished the speech," he said.

All four candidates have faced occasional disruptions during the campaign. Roh himself had eggs and tear gas grenades thrown at him in Kwangju last month and firebombs thrown near his bus in Taegu the next day.

Today's crowd was more hostile than in those incidents, comparable to the anger that Kim Young Sam encountered in Kwangju recently. But Roh played down the importance of today's disruption, as he did last month after his visits to Kwangju and Taegu.

"There are a lot of Kwangju citizens who stayed home and are not violent," he said. "The people on the street do not represent Kwangju."

Kim Young Sam, who spoke to a half-million people in Roh's hometown of Taegu on Saturday, has gotten off to a strong start in the campaign, but Kim Dae Jung supporters said today's rally will shift the momentum in their favor.

"For sure, this will be the turning point," said Kim Dae Jung's aide, Kisop Sim. "This will be the big break."

A western diplomat in Kwangju said Roh, too, may be helped by the day's events. His rough treatment may elicit sympathy, the diplomat suggested, and his cool response may win some respect.

"The road to democracy is difficult, and I am prepared for more difficult things," Roh said in an interview with foreign reporters after security guards whisked him from the rally site.

Special correspondent Peter Maass contributed to this report from Kwangju, South Korea.