South Korea's most vocal opposition figures, most of whom had spent five years in jail or had been barred from political activity, made a stunning comeback in the vote for a new National Assembly on Tuesday.

The New Korea Democratic Party's capture of 50 seats in the 276-member assembly offers evidence that the government of President Chun Doo Hwan, now in its fifth year, is resented by many Koreans.

Chun's Democratic Justice Party won the support of about the same proportion of voters -- 35 percent -- as it did in the last assembly elections, in 1981.

But opposition voters defected en masse from two comparatively moderate parties to the newly formed New Korea Democratic Party, whose candidates openly denounced Chun's government as a "military dictatorship."

Voters knew that the party, formed in January, is dominated by opposition leaders Kim Dae Jung and Kim Young Sam, two men who Chun's party maintains are symbols of an old and corrupt style of politics that South Koreans have rejected.

The New Korea Democratic Party's strong showing seems certain to keep up pressure on the government for restoration of the political rights of Kim Dae Jung, Kim Young Sam and 13 other banned politicians. The party's success was proof that Kim Dae Jung still has a significant following in South Korea.

In Seoul, political center of the country and home for nearly a quarter of its 40 million people, the new opposition party's candidates came in first in 12 of 14 electoral districts and second in the other two.

Under the South Korean system, only 184, or two-thirds of the assembly seats are filled by direct ballot. The remaining 92 seats are distributed according to a proportional formula that awards 61 seats to the party with the most votes.

Chun's party captured 87 seats nationwide, down three from its 1981 tally. It made its best showing in villages and small towns, and kept its majority in the assembly.

The other two opposition parties, the Democratic Korea Party and the Korea National Party, together won only 40 seats. Independents and splinter groups won seven seats.

The three major opposition parties together won 58 percent of the popular vote, compared to 35 percent for Chun's party. The New Korea Democratic Party accounted for half of the opposition's 58 percent.

As the most successful single party at the polls, Chun's party will be awarded 61 extra nonelective seats. Its resulting 148 seats will give it nearly a 54 percent majority in the assembly. The other parties will also receive nonelective seats, but in smaller numbers. The New Korea Democratic Party is expected to get 17 nonelective seats.

The ratio of government to opposition seats in the assembly will not undergo a major change. But the new Chun opponents will be more strident and politically experienced than the old ones.

They will clamor for more democratic freedoms, reform of labor and press laws, direct election of the president and an investigation into the Kwangju uprising of 1980, in which hundreds of people died.

"We will have to show some flexibility," said ruling party spokesman Lee Jong Ryool.

The presidency, not the assembly, is the seat of real power in South Korea. But the new party hopes it will be a base to begin a struggle that will eventually put it within grasp of real power.

If South Korea's students, who are scheduled to resume classes in March, decide to join the fray, the months ahead could see serious political tensions.

The situation Chun now faces is to some extent of his own making, the product of his decision to experiment with democracy and liberalization.

As an Army general, he seized power in 1980 during the chaos that followed the assassination of President Park Chung Hee.

He arrested or banned from political activity large numbers of dissident students, politicians, and intellectuals. But he held elections in 1981 that put him in office for a seven-year term and began easing controls, freeing prisoners and restoring political rights.

By last November, bans were lifted on all but 15 politicians. Although the two Kims were still banned from political activity and remain so, their followers were free to operate openly for the first time since 1980.

In January, they formed the new party. Later in the month, Chun called elections.

Campaigning was closely regulated, with government rules covering such details as the size and color of wall posters. The opposition complained about a stacked deck.

Yet the 20-day-long campaign saw free speech blossom, by South Korean standards, and lively public discussion of the fundamental issues of government here.

Ruling party candidates told voters they were the only hope for stability, economic growth and security from North Korea.

The opposition called for further liberalization. Despite rules limiting the content of the debate, they assailed Chun's government for "barbarous acts" and "dictatorship." Controls on the press were loosened and it began reporting some of the debate.

Much of the rhetoric was directed at Chun. Almost every ill in South Korea was laid at the door of "the man whose picture you see in the newspaper every day."

Four days before the election, Kim Dae Jung came home from two years of exile in the United States and thousands of people turned out at the airport to greet him. The party's drive gained steam.

Election day brought the largest turnout since 1960. Government officials also said the balloting was the fairest in years.

"If the election had been fraudulent," said one official, "there wouldn't be 50 deputies from the new party."

With a major victory in hand, the opposition now faces the task of ending the factionalism that has plagued it.

There is talk now of incorporating wings of the two smaller parties into the New Korea Democratic Party.

Chun, meanwhile, could come under fire from his own backers for easing controls too quickly and compromising the stability that had been achieved during the first five years of his term.

Whatever response he makes to the opposition's gain, he will do it with one eye on the international community.

Soviet Bloc countries are already suggesting that Seoul is a politically unsuitable place for the 1988 summer Olympics.