South Korean opposition leaders Kim Dae Jung and Kim Young Sam were under house arrest in Seoul yesterday, and there were reports of police detaining opposition supporters as the government increased pressure on an opposition campaign to revise the constitution.

Justice Minister Kim Seung Ky repeated government warnings that it would deal harshly with supporters of a signature-gathering campaign for a constitutional amendment to allow direct presidential elections in 1988, according to news agency reports from Seoul. The minister also threatened to send Kim Dae Jung, who is under a suspended 20-year sentence on a sedition conviction banning him from politics, back to jail if he continued antigovernment activities.

Police searched the headquarters of the main opposition party yesterday and the headquarters of the Council for the Promotion of Democracy, another opposition group, looking for signature rosters. South Korean officials in Washington said that the signature-gathering campaign is illegal and unconstitutional and that people found to be taking part face up to seven years in prison.

Kim Dae Jung, 60, who has been under house arrest since Wednesday, has not been allowed to receive visitors or telephone calls, according to State Department officials. His house has been surrounded by 1,000 policemen, Reuter reported from Seoul. Opposition leader Kim Young Sam, who shares with Kim Dae Jung de facto leadership of the New Korea Democratic Party, was arrested yesterday as he was leaving a restaurant, according to news agency reports.

The Associated Press reported from Seoul that Kim Young Sam was allowed to leave his house Saturday but that when he went to an opposition political meeting, he was turned away by a large number of police officers.

Both men have been active in the campaign, which began Wednesday, to gather 10 million signatures in favor of allowing direct election of the president. That campaign aims to amend the constitution that President Chun Doo Hwan put into force five years ago and scrap the indirect, electoral college system, which opposition critics say allows Chun to choose his successor if he honors his pledge to step down when his term ends in 1988.

The campaign has no binding legal force but represents a serious challenge to Chun because the signatures would in effect be votes against Chun, political observers said. Chun and his ruling party have rejected direct elections, arguing that they do not necessarily mean democracy and that the important thing is to bring about a smooth succession in 1988.

Although Kim Dae Jung has been under house arrest numerous times since he returned to Korea last year from the United States, this is the first time he has been cut off from visitors and telephone calls, U.S. analysts said.

Earlier this year, some analysts in Seoul suggested that Chun's tighter controls, such as his clampdown on students last year following rising protests, are fanning political turmoil just as the government seeks to show the world its best face as the future host of the 1986 Asian Games and the 1988 Summer Olympics.

In Washington yesterday, a State Department spokesman said Washington had "consistently called for the lifting of all restrictions" on Kim Dae Jung and Kim Young Sam. The spokesman said Kim Dae Jung was in good health and in no physical danger.

A department statement continued: "We believe it is inconsistent with basic democratic principles to deny citizens the right to petition their government."