The South Korean government appears to have squelched the biggest student protests in a year by arresting many antigovernment activits and flooding campuses with plainclothes policemen.

Sporadic small-scale protests continue to erupt on several of Seoul's college compuses but the wave of larger demonstrations that spread over them last week seems to have subsided under heavy government pressure and warnings that the universities may be closed down.

Foreign diplomats and Korean sources sympathetic to the students said in interviews this week that they doubted the government would permit the protests to widen to a point that would threaten its stability.

Thirteen months ago, huge student protests that swept off the campuses and into downtown avenues ended with the massive military crackdown that put then-general Chun Doo Hwan, now president, in power.

The protests this year are directed primarily against his tight rule and also against the presence on campuses of large numbers of police agents.

At Seoul National University, scene of major protests last week, students walked quietly to and from classes this week under the watchful gaze of scores of plainclothes agents. At some points, there were more agents than there were students.

Any sizable gatherings attracted agents, conspicious in their flashy sport coats and windbreakers. Student conversations were conducted in careful whispers. The only audible sound of protest was the subdued humming by several students of "We Shall Overcome," a hymn popular among American blacks in the 1960s.

The agents are drawn from three separate government organizations -- the national police, military intelligence, and the successor to the Korean Central Intelligence Agency. Only a few hundred yards from the campus are stationed two large contingents of riot policemen.

Student petitions circulated last week demanded that the university get rid of the police, but the college administrators, who are under tight government control, did not respond.

"It is a vicious circle," said Chung Won Shike, dean of the college of education at Seoul National, the country's most prestigious institution. "The students complain about the police, but without the police we cannot keep order on campus. I think that many order on campus. I think that many students now realize their presence is inevitable." He observed that under national law, police can be sent on campus without the university's permission.

It is widely believed that the government would not hesitate to shut down Seoul National and other universities if the protests grew to be as large as those that occurred last year.

There is evidence, however, that the Chun government is divided over methods of dealing with the day-to-day situation on campuses.

Last Saturday morning, Education Minister Rhee Kyu Ho, after three days of protests, hinted that a shutdown of several universities was imminent. He said in a written statement that the demonstrations would be stopped "whatever the sacrifices" and that any college that had to suspend classes had no right to exist.

About one hour later, after his warning was broadcast over radio in Seoul, his statement was withdrawn and the local media were instructed to regard it as having never been issued. No reason has surfaced to explain the turnabout, but many believe it reflected a conflict between hard-liners and moderates within the presidential office.

Spring is the traditional time of demonstrations among Seoul university students, who have repeatedly played historical roles in this country. A student rebellion in April and May 1960 led to the toppling of president Syngman Rhee. Last year's protests set the stage for the military crackdown that brought Chun to power.

This May, the government sought to divert the students' energies by staging a lavish nationalistic folk festival specifically aimed at the college generation. Although millions of persons have attended the festival, college groups have by and large boycotted it.

The demonstrations began last Wednesday and continued for three days. At Seoul National, one student leaped to his death from the roof of a five-story library building after chanting, "Down with Chun Doo Hwan." At one point, a crowd swelled to about a thousand students.

Police and the riot-control squad have arrested hundred demonstrators on Seoul campuses. The total is not known. A Seoul National administration source said about 150 students had been arrested there, with about 60 of them being released and the other 90 held for interrogation. The college has 15,000 students.

Sources in the religious community, which supports the students' causes, said that they have been unable to ascertain how many students are being held, where they are detained, or what their treatment is.

The sources, who declined to be identified because of fear of government reprisals, described the absence of information about prisoners as a persistent feature of life under the Chun administration.

They said that under the reign of president Park Chung Hee, who was assassinated in October 1979, they had been allowed to make contact with political prisoners and arrange family visits.

"The government has cut off our contacts with [jailed] students and we are not able to find out who is in and who is out," said source.

The government's harassment of Christian-oriented human rights groups has been severe, although not unprecedented. A Christian broadcasting service, once the sole source of uncensored foreign news here, has been compelled to cease news broadcasts and has been financially weakened by loss of commercial advertisements.

A prayer meeting held recently in the broadcasting company's office building was broken up by government agents. Relatives of some political prisoners were dragged down several flights of steps, a witness said.

Such incidents occurred under the late president Park's rule, but sources in Seoul's dissident community said they sense the government's harrassment and pressure have been applied more systematically than under the Park government.

The pressure "used to be turned off and on and we learned to live with that," said one source active here for more than a decade. It is "more steady" now, he said.