U.S. diplomats were negotiating tonight with between 50 and 100 South Korean students who barricaded themselves inside a U.S. Information Service library in downtown Seoul to protest American support for the government of President Chun Doo Hwan.

Although there have been student demonstrations and protests during the past six months, today's incident marks the first time that radical students have made a U.S. facility a target in their ongoing confrontation with Chun's five-year-old government. Demonstrations and clashes with police have accelerated in past weeks.

Several hundred riot police stood guard outside the building today as the students, who U.S. officials said did not appear to be armed, displayed posters from windows and attempted to drop leaflets to passers-by. The students did not harm anyone or take any hostages, U.S. officials said.

U.S. diplomats said they had told the police to stay out. No force will be used, one American said, unless the situation changes drastically and force is necessary to save lives. "I foresee a long, drawn-out siege," the official said.

Other than guards, there is no real security protection at the gray, four-story building, which also houses USIS administrative offices that the students did not occupy.

"It's the old USIS dilemma," said a U.S. official. "They're there to serve the public, and you have to give access to the public."

Initial reports said the group had distracted police guards posted outside by hurling rocks and at least one molotov cocktail shortly after noon today and then rushed up a staircase to the library, which is on the second floor.

However, U.S. officials in Seoul said tonight that there had been no violence. The students claimed that they were carrying poison pills and might use them if the police were called in.

U.S. officials characterized their protest as a "sit-down strike." The students moved furniture and books to create a barrier. Late tonight they bedded down, a U.S. official said, and talks had stopped.

The students are demanding that the United States end support to the "military dictatorship" of Chun and apologize for what they call American complicity in the bloody suppression of a 1980 uprising in Kwangju, 200 miles south of Seoul. By the official count, 191 persons were killed there, although students and others say the figure actually was much higher.

Under an agreement between Seoul and Washington, all South Korean forces are under the command of an American. U.S. officials have acknowledged that the commander in 1980, Gen. John A. Wickham Jr., released one division of South Korean troops from their regular duties to be sent to Kwangju. However, U.S. officials maintain that Wickham, now Army chief of staff, did not authorize the presence in Kwangju of South Korean paratroopers who did most of the killing.

In their discussions with U.S. diplomats today, the students also raised alleged predatory trade policies by the United States and reports that it keeps nuclear weapons in South Korea. They demanded amnesty for their occupation, a press conference and a meeting with U.S. Ambassador Richard Walker.

Students traditionally have played a pivotal role in South Korea at key political junctures. In 1960 street protests helped bring down President Syngman Rhee. In 1980, a wave of giant demonstrations ended with Chun, then an Army general, seizing power in a coup d'etat and cracking down.

South Korea has close to a million university-level students. For now, the number actively taking part in protests seems to be relatively small. However, the activists have a foothold in all major universities in Seoul and last fall set up a federation to coordinate protests.

Student activism in South Korea traditionally picks up in the spring, as the campuses fill up after winter break. This year, extra impetus appears to have come from the unexpectedly strong showing of a new opposition party in National Assembly elections held in February.

A decision by Chun to withdraw riot police from the campuses last year also appears to have made it easier to organize.

Student activists' relations with the leading opposition party, the New Korea Democratic Party, are unclear. Student leaders frequently meet with the party's de facto leaders, Kim Dae Jung and Kim Young Sam, but appear to take independent positions on many issues.

Opposition politicians generally have held that the presence of about 40,000 U.S. troops in South Korea is a stabilizing factor for the Korean Peninsula. They also have said they were counseling students against street violence in favor of action in the National Assembly.

However, the students have kept up steady pressure on the streets in recent weeks, staging small "hit-and-run" rallies that usually end with rocks and tear gas being traded. At one recent protest, police displayed a sign reading, "No rocks, no gas." There is rarely such an effort at communication, however.

Late last year, several hundred students seized the headquarters of Chun's ruling Democratic Justice Party but were evicted by riot police about 12 hours later.

Last month, hundreds of students were arrested in demonstrations commemorating the 1960 toppling of Rhee. Still more were taken in by police last week in protests to mark the fifth anniversary of the Kwangju uprising, which is a rallying point for both the students and opposition politicians.

In 1982, radicals firebombed and destroyed a U.S. cultural center in the city of Pusan. However, until recently street protests had little overt anti-American content.

Government officials say South Korea cannot afford student protests due to a threat from Communist North Korea. They also allege that some of the students are under the influence of North Korea, which is said to want to foment unrest in the South.