Students in the Chilean capital, Santiago, have clashed with police while taking part in protests over education reforms.
Police used tear gas and water cannon to break up the protests
At least 262 people were arrested during the second nationwide strike by secondary school students in a week.
Police said some of those arrested appeared to be above school age.
President Michelle Bachelet called the strike "unnecessary" after agreeing to some demands such as increased funding and free transport for some students.
But student leaders say the deal does not meet their key demands.
While the government has offered extra school funding it says it cannot give free bus passes to all - a key demand.
Secondary, or high-school, students - aged from 14 to 18 - also want a complete overhaul of an education system which they say is under-resourced and leads to huge disparities between public and private schools.
They are also calling for a university entrance exam fee to be waived.
Support for the secondary school students has been gathering apace, with university students and unions also giving their backing.
Organisers said that a million people joined Monday's strike.
Some demonstrators smashed shop windows and launched rocks and catapults at police, who responded with water cannon and tear gas.
It appeared some of the protesters were not secondary school students - either being too young or too old.
Organisers had called for peaceful action, but during the demonstrations, a number of protesters began looting stores in Santiago's main shopping district, prompting police to use water cannon and tear gas.
"Our movement is peaceful but we understand that at times the malcontent of our comrades can translate into violence," said Maria Jesus Sanhueza, a student leader.
About 20 police officers and journalists were also injured in Monday's unrest, officials say.
Chile last week saw its biggest student protests in decades as more than 500,000 pupils demonstrated.
There were violent scenes last Tuesday when riot police clashed with some of the students and several hundred people were arrested.
Television footage of young people being beaten by the security forces brought an outraged reaction from many parents and subsequently led President Bachelet to fire the head of the riot police.
Ms Bachelet, who took office in March and is facing her biggest political test so far, has offered free passes and a waiver of exam fees for the poorest students.