South Korean troops were stationed at public buildings in Seoul early today as unconfirmed rumors spread of a North Korean attempt to step up infiltration.

The troops, along with armored personnel carriers, were placed at major government buildings and newspaper offices at midnight, but were removed two hours later.

There was no public explanation for the protective forces, but government officials were quoted in Seoul as saying that they had received intelligence reports suggesting that communist forces intended to take advantage of widespread student unrest to increase infiltration in the South.

U.S. military officials said, however, that they were unaware of any evidence of a new infiltration campaign.

The appearance of troops added to the atmosphere of mystery in South Korea, already tense over repeated student protests on campus in the past two weeks. The students have demanded an end to martial law and criticized the civilian government for failing to spell out specifically its plans for a new constitution and a presidential election.

After remaining silent for two days, students at Yonsei University staged the biggest off-campus demonstration so far late today. About 2,000 Yonsei students split off from a larger crowd and charged through a university gate into the streets, clashing with riot police who responded by throwing teargas canisters. No Army troops were on the scene.

U.S. officails confirmed one military episode in the Demilitarized Zone separating the two countries. They said a United Nations command police patrol Monday evening had a brief exchange of fire with unknown forces in the buffer.

No one was injured. A check early this morning uncovered only footprints. North Korean authorities protested the shooting.

The American military officials classified the incident as "minor" and said they had no evidence that is was related to any North Korean guerrilla infiltrations.

About 3 1/2 hours after the shooting, the South Korean authorities moved small contingents of soldiers and personnel carriers to major buildings. One officer told the local press it was a "training exercise" to perfect mobilization efforts in the event of a communist attack.

Reporters were told that the evidence of North Korean infiltration plans stemmed from intelligence reports on the known positions of certain communist units. They were told that at least two brigades believed to be trained in commando tactics had disappeared and could not be located by intelligence authorities. The report allegedly came initially from Japanese sources, the reporters were told.

U.S. sources in Seoul said they knew nothing of the report. In Toyko, the Japanese Defense Agency also disclaimed knowledge.

There was speculation in Seoul that the stationing of troops at midnight may have been related to attempts to silence demonstrating students. Yesterday, reports circulated that troops would be sent onto college campuses, an action that the civilian government has tried to prevent.

During two weeks of campus demonstrations, students have clashed serveral times with riot police assigned at campus gates to prevent the demonstrators from moving out into the streets. Scores have been injured. The civilian government's education minister, Kim Ok Gill, has insisted that disciplining students be left to colleges.

Last weekend, the troubles seemed to be settled. The government canceled a series of public hearings on the proposed new constitution, a gesture to the students who believe that the constitution should be prepared and discussed by the National Assembly, not the civilian government, which contains many associates of the late president Park Chung Hee.

The students, in return, called off a major public demonstration on Seoul's streets and said they would use only "nonviolent, peaceful" means of expressing their views.

Adding to the uncertainty was a hasty special Cabinet meeting called yesterday afternoon, without the presence of interim President Choi Kyu Hah, who is in the Middle East. No results were announced.