SAN JUAN, P.R.  Months of unrest at the University of Puerto Rico seemed to be reaching a finale over the last 10 days. Scores of students were arrested or injured by riot police officers. Faculty and staff members held a two-day walkout. The president of the university resigned Friday, the police who had occupied campus were withdrawn Monday and an interim president arrived Tuesday.

But there were only three days of peace.

On Thursday morning, students blocked the stairs to classrooms in the social science department with trash cans and chairs, and also closed down the humanities department. At the social sciences building, students said only one professor had tried to get through the blockade.

The spark for the university’s problems was a budget cut that required students to pay a new $800 fee, increasing their costs by more than 50 percent.

“It is the same situation that many universities in the United States are facing,” said Miguel A. Muñoz, the interim president. “Our budget is about $1 billion, and we have been cut about $200 million. We need the $800 fee to cover the deficit, and our tuition is so low, $51 a credit, that it’s almost a gift.”