A special session of the South Korean National Assembly that convenes Monday promises some of the liveliest debate here in years and a new test for President Chun Doo Hwan's policy of political liberalization.

In the domed assembly building on Seoul's Yeoido Island, Chun's Democratic Justice Party will face a united and strident opposition that controls 126 of the 276 seats in the legislative body.

The body's deliberations are likely to be punctuated with antigovernment street demonstrations by radical students. Many of the small but persistent protests in recent weeks have taken on an increasingly anti-American tone.

It is probably a sign of things to come that the assembly's opening is already four weeks late. The cause was the opposition New Korea Democratic Party's refusal to attend until Chun agreed to restore the full civil rights of dissident leader Kim Dae Jung and free 114 persons it regards as political prisoners. But this week the party backed down and agreed to attend.

Although he continues to dominate the government, Chun has created the freest political climate in South Korea in years during recent months. He has lifted many controls on opposition activity and the press and allowed Kim Dae Jung to play a behind-the-scenes role in the new party. Kim's photo, banned from the press for five years, is now common in Seoul newspapers.

In National Assembly elections in February, the newly organized opposition party swept South Korea's cities. Its success confirmed a comeback for Kim and another prominent opposition leader, Kim Young Sam. The two men are commonly known to control the party, although they hold no formal office in it.

The New Korea Democratic Party's influence was consolidated further last month when a moderate opposition party collapsed after 30 members defected to the new group. That meant the end of the multiparty system that Chun has nurtured since shortly after he took power following the assassination of president Park Chung Hee in 1979. Chun's party, with 148 seats, retains a majority, but a small one.

Government officials say they were surprised at the opposition's strong showing in the election but have no plans to turn back the clock. "We have to continue that liberalization," said Choi Chang Yoon, secretary for political affairs in the office of the president.

"In a large industrial society, we can't expect static stability," he said.

Kim Dae Jung said in an interview that his party had agreed to attend the assembly because it had made its point that it is a different breed than the old opposition parties.

By Choi's and other accounts, party officials feared that the public was fed up with the boycott of an institution that the party had fought so hard to enter. Seoul newspapers have reported that one constituent sued to recover wages paid to assembly members during the boycott.

The government has promised to continue discussing the status of the political prisoners and of Kim, who is barred from a formal role in politics because of a 1980 sedition conviction. But Choi said amnesty for Kim is unlikely until he shows "sincere repentance" for activities that led to the conviction. Kim says the sedition charge was baseless and was brought to eliminate him as a political figure.

Choi said the prisoners are threats to national security. One of them, he said, was convicted of the 1982 firebombing of a U.S. cultural center in Pusan in which one person was killed. Lenience might be possible through normal judicial procedures, he said, but their future is not politically negotiable.

The new party maintains that 114 are "prisoners of conscience," government critics against whom anticommunist and other criminal laws have been used improperly.

One of the first orders of business for the assembly will be confirmation of the appointment of Chun's new prime minister, Lho Shin Yong. Lho came from the top job at the National Security Planning Agency, as the Korean Central Intelligence Agency now is called, and the opposition is expected to try to block the appointment.

Other issues the opposition is expected to raise are constitutional changes to require the direct election of the president, reform of the labor and press laws and alleged election law abuses by the government in February.

In the meantime, Chun's government is facing continuing pressures on the streets from some radical student and labor groups. To date, the protests have been small, but some diplomats here see a potential for them to grow rapidly.

Last month, during demonstrations to commemorate the anniversary of a 1960 student uprising, violence broke out, and riot police arrested hundreds of protesters. Many of them were protesting a visit to Washington by Chun. More demonstrations are expected later this month on the anniversary of the 1980 antigovernment uprising in Kwangju in which hundreds of persons died.

Protesters have been distributing pamphlets and posters attacking "foreign influence" and the U.S. role in South Korea. At the same time, police have confiscated "subversive" literature from Seoul bookstores.

Western diplomats here cite three cases of physical attacks on Americans in Seoul in recent weeks that may have been politically motivated. In one, a rock shattered the windshield of a car carrying a U.S. general past a student demonstration. Choi said the incident was random and not specifically directed at the general as an American.