Several hundred police and soldiers ringed the campus of Beirut Arab University on Monday as students returned to classes for the first time since a political spat in the cafeteria mushroomed into street riots that killed four people and inflamed Sunni-Shiite tensions. Campus security guards searched all students at the entrance and denied access to those without student IDs. Troops in armored personnel carriers and police officers watched the students arrive in a show of force meant to deter troublemakers. The clashes of Jan. 25, which quickly spread from the campus to the streets of Tarik el-Jadideh district and beyond, was the worst case of sectarian violence since the 1975-90 civil war. Muslim Sunnis fought Muslim Shiites with rocks, sticks and even guns, leaving four people dead and dozens injured. Scores of cars were trashed and some were set ablaze. In the evening, the army imposed a rare nighttime curfew and leaders of both communities appealed to their supporters to withdraw from the streets.