Defense Secretary Harold Brown, arriving here amid new antigovernment unrest, assured South Korea today that the United States would come to its defense "promptly and decisively" if it is attacked by North Korea.

Brown, coming to Seoul for a defense meeting, arrived on the same plane with U.S. Ambassador William Gleysteen, who was recalled two weeks ago as a gesture to American didapproval with the South Korean government party's removal of opposition leader Kim Young Sam from the National Assembly.

A few hours after they arrived, the civil unrest sparked by the expulsion surfaced as martial law was imposed in Pusan following tow nights of rioting by students. The port of Pusan, 200 miles southeast of here, is the second largest city in South Korea.

The most violent in many years in this counrty, the Pusan disturbances that began Tuesday night were in part expressions of public support of Kim.

Public anger about Kim's expulsion led to the opposition pulling out of the assembly and was partly behind the Pusan protest last night. About 3,000 students from two universities reportedly staged violent protest and were joined by some citizens on the streets. About 200 were arrested, and about 50 police were injured. The last riots so violent were mounted in 1964 to protest the normalization of relations with Japan.

Reports from Pusan said the students surged into a downtown area, chanting antigovernment slogans and destroying at least 11 police boxes. Some of the slogans were in support of Kim, the leader of the New Democratic Party who was expelled because of his remarks criticizing the government.

However, some reports said the demonstration began not as a gesture of support of Kim but as a purely college-oriented protest against the stationing of police agents on campus.

The government was alarmed enough to send Home. Minister Koo Ja Choon to Pusan to see that order was restored. He promptly fired the police chief of Pusan and promised "stern measures" to prevent a recurrence. Martial law followed.

Again tonight more than 3,000 students in Pusan demonstrated against the government, burning or destroying 21 police substations and 18 police cars. They also attacked several government offices, radio stations, and the offices of a pro-government newspaper.

Under the martial law edict, all news of the uprising was ordered censored and the curfew was extended by an hour to run from 11 p.m. until 4 a.m.

Armed troops were moved in to guard government offices and other important facilities, and the military was authorized to arrest citizens without warrants and to ban all unauthorized public gatherings.

It was the first time since 1972 that martial law has been imposed anywhere in South Korea. That was the year of protests against the new authoritarian constitution announced by President Park Chung Hee.

Kim's ouster caused another in a series of controversies between Park's government and the United States. Following Gleysteen's recall and return, it is considered possible that Brown will deliver some further message of disapproval Thursday when he meets with President Park.

However, U.S. Defense Department officials said here tonight they expect to keep the political issue "insualted" from the defense talks, park of a regular annual review of American and South Korean readiness.

The usual South Korean doubts about the U.S. commitment were heightened recently by some off-the cuff remarks of U.S. Ambassador to Japan Mike Mansfield. In a Tokyo news conference last week, he implied that he does not consider South Korea within the Asian defense perimeter of the United States. His remarks caused a stir within the South Korean government, which is wary of any sign of a weakening U.S. commitment, even though President Carter has dropped earlier plans to withdraw American troops.

Brown's remarks today apparently were designed to allay those fears. He said the United States will maintain "strong forces in Korea and throughout the Pacific. This will enable us to maintain stability in the region and deter acts of hostility or adventurism.

The two days of militaty talks here are the first since intelligence analyses revised estimates of North Korean troop levels to show increased strength. The United States estimates the communist troop strength is now more than 600,000 men, an increase of more than 50,000 more than previous estimates.

South Korea has made even higher estimates. Last weekend, Defense Minister Ro Jae Hyun asserted that North Korea fields an army of 720,000 men and has increased its armored strength by 30 percent.