Thousands of South Korean students took to the streets today in the biggest antigovernment demonstrations here in years, and military forces moved in to protect key government buildings.

An estimated 30,000 students marched through downtown Seoul, clashing with riot police and chanting antigovernment slogans in what was by far the largest demonstration since the protests began two weeks ago. Demonstrations also took place in provincial cities.

The chaotic scenes in Seoul revived speculation that South Korean military forces would intervene, acting under martial law authority they have held since President Park Chung Hee was assassinated last October.

Scores of students were injured in the confrontations that began at noon and continued late into the evening. Downtown Seoul was in bedlam much of the day as riot police repeatedly dispersed large crowds with salvos of tear gas.

In the last two days, the students have stepped up pressure on the government to lift martial law and spell out a specific timetable for amending the constitution and holding a new presidential election. They also are demanding the immediate ouster of two prominent officials.

Several hundred troops from a unit garrisoning the capital moved into place tonight around government buildings, the first time military forces have been displayed against the students during the current unrest. They were backed up by a dozen armored personnel carriers but had no direct conflict with the students.

Early yesterday morning, during curfew hours, they had surrounded several buildings for two hours in what was described as a training exercise.

Protests also were reported today in the major provincial cities of Taegu, Pusan, Chun Ju, and Kwang Ju. More than 10,000 students marched in Taegu, burning a riot police bus and destroying a police substation.

[About 2,000 rock-throwing students battled police again Thursday morning in Kwang Ju, United Press International reported. No injuries were immediately reported.]

Meanwhile, the civilian government again appealed for order but appeared unprepared to take any forceful action to curb the demonstrations. There were no mass arrests and the approximately 300 students seized by police in the early evening were quickly released.

Education Minister Kim Ok Gill, who had sponsored several compromises to preserve campus peace this spring, asked the students to remain on campus and termed the street protests illegal.

The Cabinet met twice today under the leadership of Prime Minister Shin Hyon Hwach, who is in charge while interim President Choi Kyu Hah is traveling in the Middle East. Despite rumors of a new government policy, the Cabinet announced no new measures to deal with the unrest.

Students began demonstrating two weeks ago, but had kept most of their protests on campuses, marching out the gates sporadically to clash with riot police. But last night, about 2,000 marched into the streets, and overnight careful plans were laid for today's massive demonstrations.

The issues originally were campus complaints about military drill, but they have escalated this week to include broad political demands.

The government has said it will not lift martial law until peace is restored. It has generally implied that a presidential election will be held under a reform constitution next spring, but has refused to announce a time table.

Students and opposition political leaders insist the government is delaying an effort to restore some of the authoritarian politics of Park's regime and they complain the military has attained extensive behind-the-scenes political power.

The protest is increasingly centered on two figures -- Prime Minister Shin and Lt. Gen. Chun Doo Hwan. Chun is a powerful figure in the military clique that seized power within the armed forces last December and has since been given the key post of acting director of the Korean Central Intelligence Agency.

During today's protests, students carried placecards demanding the ouster of both Shin and Chun. One group carried a red funeral flag and coffin bearing the label of "Yushin Remnants." Yushin was the Korean name of Park's authoritarian system.

The demonstrations were the largest since the student uprising of April 1960 which toppled the government of President Syngman Rhee and led eventually to a military coup, from which Park emerged as a strongman president.

Following a schedule worked out at all-night campus meetings, the students moved toward downtown Seoul in four waves early this afternoon, chanting, singing and carrying placards and banners.