South Korean students clashed with police near two campuses tonight in the first violent confrontations since a wave of demonstrations began early this spring. At least 38 people were injured.

The protests were the first off-campus demonstrations since martial law was imposed in October after the assassination of president Park Chung Hee.

They raised the level of tension in South Korea, where a wave of labor strikes has spread in the past 10 days. The martial law authority warned yesterday that stern measures would be taken if social unrest continued.

The civilian government, hoping to avoid intervention by military authorities, tried for weeks to negotiate settlements with student groups and arranged several compromises. Until tonight, students kept their demonstrtions peacefuly within campus boundaries.

At lease 28 students from Sungkyunkwan University here were injured when riot police blocked an off-campus march by 1,500 students protesting military training and other government policies.

At Taejon, 150 miles south of Seoul, about 3,000 students clashed with police and held a two-hour sit-in at a railroad station. Seven students and three police officers were injured.

Students at Seoul National University here also marched toward the campus gate but turned back without a clash in the face of a large contingent of riot police.

The students protested a variety of government measures, including the military obligations imposed on them while in college. Earlier today, in the latest compromise, the Education Ministry had agreed to reduce their weekly campus military drills from four hours to two hours.

But the ministry has been unable to arrange a compromise that would eliminate a requirement that all freshmen must drill on a military base for a 10-day period.

That was the main issue at Sungkyunkwan University. Eighty-seven freshmen who had refused to report last month for their 10-day drill had been ordered to be inducted immediately into the military service and the march was protesting their induction.

As they left the campus they began throwing stones at a large group of riot police officers who responded with tear gas. The students retreated but a second confrontation began two hours later, and they finally retreated into the campus for a sit-in.

The 3,000 students at Taejon's Chung Nam University marched and clashed with police to underline their demands for dismissals of professors they said represented the old Park government. They also called for lifting of martial law and the right for freer campus demonstrations.

At Seoul National University, students held a rally and marched to a main gate but decided not to risk a clash with riot police stationed outside.

The Seoul National students were demanding a lifting of martial law, free press coverage of political events, the resignation of some college managerial personnel and full rights for South Korean workers.

The off-campus demonstrations broke a fragile truce that had been established under the leadership of Education Minister Kim Ok Gill. The martial law command has remained out of the controversies.

The martial command has repeatedly warned against illegal protests and on one occasion handed down severe jail sentences to a number of dissidents who staged a protest march in downtown Seoul.

At least 25 labor strikes, two of them accompanied by large-scale violence, have spread across the country in the past 10 days. Strikes also are illegal under both martial law regulations and the country's national security law.

Most of the strikes were peaceful as company officials granted large wage increases to avoid trouble.