More than 150 South Korean dissidents have been arrested in Seoul since the military takeover three weeks ago, and about 400 more are in hiding to avoid being captured, according to reliable sources.

Hundreds more are said to have been seized or have gone underground in other parts of the country, dissident sources have estimated.

The numbers include ministers, priests, lawyers, professors and students, many of whom had been freed from prison only a few months ago at the beginning of a brief period of liberalization in this country.

The blackout on information for so long a period is unusual. During the periodic police sweeps common under the late president Park Chung Hee, relatives of those taken usually were able to obtain information within a few days and were frequently permitted to visit them in their cells.

The relatives said they feared many of those taken were being beaten, pointing to the experience last November when martial law authorities cracked down on a group of protesters in downtown Seoul. When released, many of them described severe beatings in two detention centers operated by military authorities.

The latest wave of arrests began shortly before midnight on May 17 when a group of generals seized control of the government and proclaimed a new, expanded version of martial law. Judging from several accounts, the arrests were made simultaneously about a half hour before midnight when the proclamation was announced, apparently to ensure that the raids would be a surprise.

In one case, the Rev. Lee Hae Dong, pastor of the Protestant Hanbit Church, was arrested at 11:40 p.m. by four plainclothes officers who led him away after searching his home for more than three hours. His wife said today they took away photographs and papers belonging to her husband.

Lee had been imprisoned for more than a year after joining other religious leaders in issuing a 1976 antigovernment manifesto. He was jailed again for 10 days last November, then suddenly released.

Widespread searches of homes of dissidents have continued in the military's attempt to round up those who managed to flee their homes on May 17.

Mrs. Lee said that on May 21 about 30 plainclothes surrounded her house and searched it again, looking for someone. A prominent lay member and four students active in the Hanbit Church were discovered and arrested on May 24, she said.

The home of a prominent journalist, Park Chong Mun, was searched this morning. He had fled when the arrests started. He was one of several staff writers of a leading newspaper, Don-a Ilbo, who had rebelled earlier against government censorship. According to family friends, his mother was told by the searchers be had been killed in an automobile accident, but friends said he is still in hiding.

Reports trickling into Seoul from other cities indicated the pattern of arrests was similar throughout the country and is continuing, but no nationwide compilation is available. The dissident community here believes that the total number of those arrested or in hiding runs into the thousands.

The government has named only 26 persons arrested in Seoul, including a number of political figures and former government officials never involved in dissident activities.

The roundup appears to be directed at almost anyone previously in trouble with the Park government. The majority are persons arrested in the latter years of Park's rule and suddenly released early this year as Park's successor, President Choi Kyu Hah, opened a period of relative freedom and dissent.

Among those seized was Yonsei University Vice President Kim Dong Gil, the brother of a former education minister, and three prominent professors: Han Wan Sang of Seoul National University, Suh Nam Dong of Yonsei and Lee Moon Yung of Korea University.

Religious figures arrested included a Presbyterian minister, the Rev. Moon Ik Hwan, who had been released in January; and two Catholic priests, Ham Se Ung and Kim Seung Hoon. Also seized was Han Seung Hun, an attorney who represents the Seoul branch of Amnesty International.