The test had been suspended twice since November amid widespread social unrest in Chile that has left more than 27 people dead
University entrance exams to be taken by 300,000 students around Chile were disrupted in some cities on Monday by fresh protests over inequality and elitism, with some students blocking access to test sites and burning exam papers.
The authorities suspended the university selection test - known locally as the PSU - in 64 of more than 700 exam centers around the country, citing in a statement the safety of students, staff and exam materials.
The test had been suspended twice since November amid widespread social unrest in Chile that has left more than 27 people dead, thousands injured and thousands more arrested.
With the unrest dwindling to a handful of smaller protests each week, the test had been rescheduled to this week, only to face calls for a boycott by student union groups that claim the admissions system privileges those who attend better schools, in better areas and have better resources to prepare for it.
"We will continue to fight against market education and for a country where poor and working-class children can study without competition or segregation," the Coordinating Assembly of Secondary Students (ACES) said in a statement on Twitter.
Students protested outside exam centers in cities around the country, including Santiago, the capital, the coastal city of Valparaiso and Calama to the north. Students carried banners calling for a more equitable education system.
In some cases, students blocked access to the sites, disrupted the exams, vandalized classroom furniture and burned exam papers. Eighty-one people were detained for damage and public disorder, the Interior Ministry said in a statement.
Education undersecretary Juan Vargas said those affected by the suspensions would be able to complete the test later.
"That group of young people who have been affected will have a solution, an alternative, to take the PSU," he told reporters in Santiago.
In recent years protests by Chilean students over the country's highly privatized education system have made global headlines and brought pledges of reform including making higher education free of charge, improving quality and widening access to more people.
Since October, Chile has been rocked by broader protests that started over a hike in public transport fares and broadened to include calls for better pensions, healthcare and education.