Protests by thousands of striking high school students demanding better public education resulted in at least 140 arrests, left 18 riot police injured and caused damage to public and private property, the government of the Santiago metropolitan region and Chile's Carabineros militarized police force said.

The national student strike followed three weeks of protests that included school occupations punctuated by police operations to evict the occupiers.

In a statement to the media, the authorities said 39 of the detainees were adults who were accused of disorderly conduct and assaulting police officers during Thursday's protests.

"The (student) leaders did not keep their word not to block traffic and forced detours to be set up on more than 40 roads, as well as causing damage and destruction to public and private property," the head of security for the Santiago metropolitan region, Gonzalo Diaz del Rio, said.

He said three riot police had been hospitalized with serious injuries and that students from the prestigious, downtown Instituto Nacional, located two blocks from the presidential palace, set materials on fire at the establishment.

According to estimates by the Carabineros, nearly 8,000 students took part in 14 unauthorized marches, in which the protesters fanned out to Santiago's communes to present their demands to several mayors in greater Santiago.

Clashes with police erupted at the end of marches in the capital's downtown and near the mayor's office in the Santiago commune of Providencia.

In downtown Santiago, confrontations broke out after 3,000 youths who had assembled at the Plaza de Armas sought to march along Bernardo O'Higgins avenue and some nearby streets.

Police warned the protesters not to interfere with traffic and eventually used water jets and tear gas to disperse the youth, who responded by throwing rocks and other projectiles.

Similar violence occurred in Providencia, although in that case police rushed at some 5,000 youths congregated outside the local government offices in the commune, preventing them from delivering their demands to the mayor.

"What's clear is that it's a minority - headed by leaders who do not represent millions of students and (student) representatives - who are trying to impose their demands. That attitude isn't democratic nor does is further the goal of improving Chilean education," Diaz del Rio said.

Student leader Eloisa Gonzalez, however, justified Thursday's protests, noting that the multiple simultaneous marches marked a new strategy by the high school student movement."

"We're extending it out to the communes and other sectors that hadn't participated in these types of demonstrations," she said.

Chilean students took to the streets in large numbers more than 40 times in 2011 to denounce a highly stratified education system that funnels state subsidies to private institutions even as public schools in poor areas struggle.

The protests have continued this year and another nationwide mobilization is planned for next Tuesday.

Chile's public schools and universities were neglected by the 1973-1990 dictatorship of the late Gen. Augusto Pinochet, who embraced doctrinaire free market policies.

Private schools mushroomed under the military regime and the trend continued after democracy was restored, even during the 1990-2010 tenure of the center-left Concertacion coalition.

The students want public primary and secondary schools to be administered centrally, not at the level of individual municipalities, as is currently the case, as well as the elimination of school fees.

The movement also demands an end to for-profit universities and a reduction in the high cost of college, which forces many students to take on large debt.

Piñera, a right-wing billionaire, has taken some steps to make college more affordable for low-income students and is now asking Congress to pass a tax reform bill that would generate as much as $1 billion in additional education funding.

Critics dismiss that figure as woefully inadequate. EFE