SEOUL, DEC. 21 -- Defeated opposition leader Kim Dae Jung today apologized to the South Korean people and said his inability to form a common front with rival opposition leader Kim Young Sam allowed the ruling party to retain power in last Wednesday's election.

The apology represented an about-face for Kim, who earlier said that governmental fraud was so widespread that even a unified opposition candidate would have lost.

In advertisements published on the front pages of afternoon dailies today, Kim continued to decry what he called governmental cheating but said the division of antigovernment forces also contributed to their loss.

"I sincerely apologize and realize that my responsibility is so great that I cannot bear it," Kim said in the advertisement. "The cause for the failure to defeat the regime lay in the failure to achieve a single candidacy."

Ruling party candidate Roh Tae Woo was elected with 36.6 percent of the vote in South Korea's first direct presidential election in 16 years and is scheduled to succeed President Chun Doo Hwan Feb. 25. The two Kims together polled 55 percent, with Kim Dae Jung in third place.

Both Kims initially blamed their loss on governmental manipulation and urged rejection of the results. Many people, including opposition supporters, said they felt angry that the Kims refused to accept responsibility after breaking their promise to field a single candidate.

Kim Young Sam apologized last week in a statement that went on to place most of the blame on Kim Dae Jung. The two longtime opposition leaders have not spoken to each other since their loss.

Small demonstrations against the election results continued in several cities, mostly in Kim Dae Jung's home province of Cholla. In the provincial capital of Kwangju, a few students tried to throw firebombs at the U.S. Information Service office, but police chased them away and there was no damage.

Kim Dae Jung's partisans in Kwangju have attacked Americans for what they see as pro-Roh bias in the U.S. government and press.

The National Coalition for Democracy, the dissident group that spearheaded the fight for the Dec. 16 election, said today it had documented widespread fraud. The coalition said it had received 791 reports of irregularities during voting and another 256 during vote-counting.

But many young politicians in the opposition camp continued to express disappointment with the two Kims' policy of rejecting the election results. Chung Dae Chul, spokesman for Kim Dae Jung's party, resigned.

"Now we must get free of the illusion that we could have won the election but for the election-rigging and fraud," Chung said.

As young members of the opposition called for new leadership, Kim Young Sam's party warned that it will punish those who "break party unity."

His party also "deeply" apologized to the people. "We conclude that we lost primarily because of the split," a party spokesman said.