Police in Chile have used tear gas and water cannon to break up a protest by thousands of students in Santiago.
It was the first big demonstration this year by Chile's student movement, which held months of mass protests in 2011.
They are demanding free, high-quality public education for all, as well as the reinstatement of students excluded from school for protesting.
Meanwhile, the government is invoking a security law to tackle separate protests in the southern Aysen region.
The student demonstration began as a peaceful march in the centre of the capital.
Clashes broke out as hundreds of protesters tried to break through police lines outside the Education Ministry.
Masked youths set up burning barricades and threw stones and petrol bombs at riot police.
At least 50 people were arrested, the authorities said.
The dispute over educational reform remains unresolved despite months of mass protests backed by teachers and trade unions as well as students.
The conservative government of President Sebastian Pinera has offered some concessions, but has refused to accept student demands for a radical transformation of the school and university system.
Correspondents say more protests and clashes are likely now the summer holidays are over.
Meanwhile, the government says it is invoking the controversial state security law to combat disorder in the Aysen region in Patagonia in the far south of Chile.
The remote region has seen weeks of protests by local people who say the government is ignoring their needs.
Their demands include fuel subsidies to compensate for high prices, a regional minimum wage, and more generous quotas for local fishermen.
Government ministers sent to negotiate with regional leaders have so far been unable to quell the unrest.
In the latest disorder, a police bus and riot vehicle were set on fire during clashes with protesters, the authorities say.