Protesters threw stones, flares and petrol bombs at police who responded with teargas.
The incidents occurred during a march by students to parliament, protesting against a draft bill changing university entrance requirements.
Reuters witnesses saw at least three petrol bombs hurled by demonstrators, who also threw flares. Police responded with rounds of teargas.
The incidents ended almost as rapidly as they started and the central square soon opened up for traffic.
Reforms wanted by the education ministry include curriculum changes in the final year of high school and alterations to what are now uniform entrance exams to university, irrespective of the chosen field of study.
High school students say the changes will increase the exam burden and create a system which discourages critical thought.