South Korean police have again confined prominent dissident Kim Dae Jung to his house in Seoul, the latest step in a crackdown against certain government opponents.

Police guards appeared outside Kim's house at 7:30 a.m. today in an apparent effort to prevent him from attending a convention of the opposition New Korea Democratic Party, which starts Thursday. It is unclear if the detention will continue after the convention ends.

Kim also was held under house arrest briefly after his return from exile in the United States in February.

Reached by telephone at his home, Kim said he had received a visit from his neighborhood's police chief. "He told me, 'You are prohibited to take part in party affairs, but you have repeatedly violated the law,' " Kim said.

A government spokesman said this confinement is a "preventative measure" to stop Kim from violating the law. Kim is barred by law from participating in politics because of his 1980 conviction on sedition charges in a military court.

Police did allow Kim to receive visitors today, including many delegates to the convention.

Police also imposed house restriction on Kim Sang Hyun, vice chairman of the antigovernment Council for the Promotion of Democracy and a close associate of Kim's.

Late last year, President Chun Doo Hwan, who heads the ruling Democratic Justice Party, lifted many restrictions imposed on opposition politicians and allowed them to form the New Korea Democratic Party. In National Assembly elections in February, it became the country's main opposition party.

Although Kim and fellow opposition leader Kim Young Sam hold no formal offices, they are commonly believed to direct the party.

The government seems to have begun reevaluating its stance since radical students seized the U.S. Information Service library in Seoul in May to press demands for changes in the government. The students later left peacefully.

Prosecutors brought criminal charges against 20 of 73 students who allegedly were involved in the takeover. They are being tried in small groups because chanting and jeering interrupted a joint trial earlier this month.

On June 29, hundreds of plainclothed police staged surprise raids on nine university campuses and reportedly arrested 66 persons. Police officials were quoted as saying the action was taken to stop the campuses from being turned into bases for political struggle.

On July 18, the government declared a student group named Sammintu to be a procommunist radical organization and arrested or charged 86 persons said to be its members. Many of the students in the library takeover were said to be members.

The police said Sammintu was backed by "impure" forces, a term that normally means North Korean or North Korean-controlled.

The government also noted the group's strident attacks on the United States, which maintains about 40,000 troops here. The students blame the United States for supporting what they call a military dictatorship.

The government contends that the campus radicals are a tiny, violent minority that is threatening society. "When the police raided the campuses," the government spokesman said, "they found hard evidence that their activities are beyond mere student protests." Molotov cocktails and subversive literature were discovered, he said.

Student activists, however, say the police action is a response to what they call their movement's growing momentum and appeal to the public. "It is inevitable that the government will try to block us," said one student. "They are beginning to feel their lack of power."

The arrested students have received some support from the opposition party members, who call their arrest politically motivated.

Support has also come from within South Korea's large Christian community. The National Council of Churches in Korea issued a statement charging that the government was trying to divert public attention from society's true problems, such as poverty, and was not tolerating dissenting views.

Meanwhile, in recent months, the party has waged a noisy, inconclusive legislative battle with Chun's Democratic Justice Party.

The ruling party has refused to consider calls for revisions in the constitution to provide for direct election of the president. It also has consistently refused to restore the political rights of Kim Dae Jung, saying he must first show "repentance" for his past acts.