SANTIAGO, Chile - Police on Thursday used tear gas and water cannons to disperse hundreds of rock-throwing high school students who repeatedly blocked traffic on Santiago's main avenue.

The government said 159 students had been detained shortly after noon and some clashes continued.

The demonstrations came on a date often marked by violence by far-left groups commemorating what they call "The Day of the Young Combatant," honoring two young brothers killed by police in a 1985 protest against the 1973-90 dictatorship of Gen. Augusto Pinochet.

On Thursday, the judge investigating the killing of Eduardo and Rafael Vergara filed homicide charges against one active and three retired police officers. A attorney for the retired officers, Mauricio Unda, said that they acted in self-defense during the protest.

The students' motives for protesting Thursday were not clear. One protester, who identified himself only as "Simon," his face covered by a white handkerchief, said only that they were demonstrating "for the situation and for our rights."

The mostly small protests, however, were far smaller than the well-organized marches of May 2006, when up to 700,000 high school students took to the streets to demand improved schools, lower public transportation fares and educational reforms.

The government announced Wednesday that it was mobilizing 4,000 police to prevent the violence that often breaks out on "The Day of the Young Combatant."