A newly organized opposition party swept South Korean cities in yesterday's election to take 50 seats in a new National Assembly and become the prime force against the government of President Chun Doo Hwan.

The New Korea Democratic Party, composed primarily of politicians affiliated with dissident leaders Kim Dae Jung and Kim Young Sam, took 28.9 percent of the popular vote, with 92 percent of the ballots counted.

Chun's ruling Democratic Justice Party lost two seats to win a total of 88 but still retained its majority in the assembly. It got 35.5 percent of the total vote.

Minor changes in the results were possible as final votes come in from South Korea's isolated island communities.

The vote left the proportion of seats held by the government and the opposition basically unchanged. But it was significant for sealing the comeback of opposition figures who were banned from political activity until November of last year.

Yesterday's voting followed a 20-day campaign that was tightly regulated but nonetheles included the freest debate of political issues since Chun, a former Army general, took power in 1980.

Two other opposition parties, the Democratic Korea Party and Korea National Party, suffered serious setbacks in the vote, which drew an 84 percent turnout, the largest since 1960.

Under the South Korean system, each of 92 districts elects two people to the assembly -- the candidates placing first and second in the number of votes. The government party did best in rural areas, which traditionally are more conservative than the cities.

The New Korea Democratic Party overpowered Chun's party in Seoul and many provincial cities, which are the center of political power here.

New Korea Democratic Party candidates, many of whom had publicly condemned Chun's government as a military dictatorship, came in first in 12 of Seoul's 14 districts and second in the other three. In Pusan city, the government party lost altogether in three districts; its candidates did not place either first or second.

The chairman of the government party, Kwon Ik Hyun was quoted this morning as saying that his party "now knows the will of the people."

Kim Dae Jung, who has been under house arrest since his return from exile last week, said he watched the returns on television.

He and 14 other politicians are prohibited from any political activity and thus were not allowed to vote.

"Despite New Korea Democratic Party's unfavorable conditions, it has succeeded in winning a great victory," Kim Dae Jung said.

Under the South Korean system, the government party, as the largest vote getter, will be awarded 61 nonelective seats on the grounds that one party should possess a working majority.

Its resulting 149 seats will give it about a 54 percent majority in the 276-member assembly. The New Korea Democratic Party, will get 17 nonelective seats, for a total of 67.

Because of the tight control the government exercised during the campaign, opposition parties charged that a fair vote was impossible.

Election day had been declared a holiday to increase the turnout, and voting was reported to be generally orderly.

Clusters of men and women in traditional dress marched along country roads toward the polling places, which in most areas were in elementary schools.

In the cities, factory workers and students crowded into polling places. The weather was generally fair.

At a polling place in a classroom in Kwangju city yesterday, voters stepped inside curtained booths to mark their ballots, then dropped them into a sealed metal ballot box.

Observers from the five parties that fielded candidates in the district watched the proceedings from a special table and at 2 p.m., with four hours of voting to go, reported no irregularities.

The government has announced that it is investigating about 460 people for election law violations, all but a handful of them in the opposition camp.

The vote was a key test for the noncommunist opposition, whose spirits have been buoyed by the return of Kim Dae Jung from two years of exile in the United States.

Opposition leaders decided to take part despite their allegations that the election law and Chun's authoritarian rule would stack the vote in favor of Chun's party.