MEXICO CITY -- (AP) -- Graffiti covers the walls, and murals mocking school
 officials adorn the corridors of Latin America's largest university. A university
 police car is parked outside one building, its normal blue-and-white covered in
 psychedelic motifs.

 National Autonomous University's fall semester was to begin today, but has been
 delayed indefinitely for most students.

 Silence looms over the sprawling and normally bustling main campus, four
 months after a student-led strike brought classes to a halt.

 The calm scene stands in stark contrast to recent off-campus clashes with
 Mexico City police as students protesting tuition hikes at the university -- also
 known by its Spanish initials, UNAM -- tried to prevent incoming freshmen from
 registering.

 ``This is a decisive moment because if the conflict between students and officials
 goes on, it will have disastrous consequences for the UNAM, for students and
 professors,'' said literature professor Maria Stoopen.

 In recent weeks, conflict has been more common than negotiation with little hope
 for an immediate resolution. In the most recent confrontation, 150 riot police were
 called out Friday after 50 striking students formed a human chain to block a
 registration center north of the city. No violence was reported.

 UNAM officials said 195,000 of the expected 268,000 students had been able to
 register for the fall semester. But on Friday, they announced that 45,000 students
 -- mostly those who use off-campus facilities -- would begin the semester on time.

 Students have barricaded the campus, and it is unlikely they will be forcibly
 dislodged, in part because of history. One of the darkest moments in Mexico's
 recent past came when soldiers fired on anti-government student protesters in
 1968, killing an estimated 300 people.

 The present strike began April 20 to protest a decision by university President
 Francisco Barnes de Castro to raise tuition for the first time in more than 50 years
 from a symbolic 2 cents to $160 a year.

 The Mexican constitution guarantees free public education to all, but UNAM
 officials argue that doesn't include higher education.

 As the strike dragged on, Barnes agreed to forget about the tuition increase. But
 by then the students were fired up and pressed other demands, such as greater
 student participation in running the school.

 The spring semester was suspended, and with negotiations stalled on the
 students' new demands, professors have begun to take the initiative to end the
 strike.

 Teacher and students sought a basic agreement Thursday that would give
 students and professors a greater say in the university's academic policies. They
 failed to strike a deal.

 ``We are in agreement with the students that the university is in need of an
 overhaul,'' said history professor Cristina Gomez. ``But we disagree on the means
 to reach that end.''

 Though some students are fed up with the strike, sophomore Yaokoatl Chavez, a
 Latin American Studies major, says the strikers are resolute in their demands.

 ``We are worn out, tense and tired -- but not discouraged,'' he said.