At least 100 people were injured and 262 arrested here in rioting and demonstrations by school pupils, during which police deployed water cannon and tear gas, said media reports.
The riots took place on the fringes of the largely peaceful protests by tens of thousands of pupils rallying for educational reforms and free public transportation for students.
A minority of the demonstrators erected barricades, threw paving stones at police and looted stores. Many of their fellow protesters tried to stop them.
The pupils' ACES organisation called for the more than 1 million schoolchildren and university students to participate in a nationwide "social strike" for reforms of the educational system established during the 1973-1990 dictatorship of Augusto Pinochet.
Outside Santiago, the protests proceeded peacefully.
President Michelle Bachelet has already rejected the student demand to ride buses without paying. She offered discounts but said the government could not afford to issue free passes to pupils.
However, she promised to allow pupils to participate in planned school reforms.
The protesters are seeking a rollback of the changes made during Pinochet's regime as well as a repeal of fees for university entrance exams.
Some parents and university students have joined the rallies by school students, who have been demonstrating for more than a month.
{{/usCountry}}Some parents and university students have joined the rallies by school students, who have been demonstrating for more than a month.
{{/usCountry}}The protests have presented the biggest challenge of Bachelet's 3-month-old presidency.