Police have clashed with student protesters in Santiago and arrested at least 455 people across Chile duringcommemoration protests, officials say.

Tear gas and water cannons were used to disperse hundreds ofhigh school students throwing rocks, who repeatedly blocked traffic on Santiago's main avenue,police said.

According to Chilean police, more than 100 police officerswere injured in the clashes.

Adriana Delpiano, Santiago'sgovernor, said demonstrators on Thursday were demanding better publictransport.

The annual "Day of the Young Fighter" protestcommemorates the 1985 police killings of two students during the presidency ofAugusto Pinochet, Chile'slate military ruler.

The day is often marked by violence and the night's protestswere typical of the date.

Transportation anger

Thecommemoration called this year was marked by anger over Santiago'sreorganised public bus system, which users say causes undue waiting at stops.

One protester, who identified himself as Simon, his facecovered by a white handkerchief, said that they were demonstrating "forthe situation and for our rights".

Policedeployed 4,000 officers and a dozen armoured personnel carriers around the cityfor the event.

Some hoodedprotesters threw Molotov cocktails in the south of Santiago,and the capital's universities closed early on Thursday to prevent a spill overof the protests on to their campuses.

Most shops in Santiagohad closed by mid-afternoon.

Felipe Harboe, Chile's deputy interior minister, toldAgence France-Presse: "During the day we had a national number of arreststotalling 475."

TheAssociated Press reported Harboe as saying 455 demonstrators had beendetained at dusk on Thursday.

Harboe alsosaid that most of those arrested in the afternoon were minors who were causingtrouble near the presidential palace, La Moneda, in the centre of the capital.

Night violence

Bynightfall, the violence had spread to poor neighbourhoods, where protesters setup barricades and exchanged gunfire with police officers, according totelevision footage.

Rocks thrown by demonstrators broke windows at the intensivecare unit of the Santiago Workers Hospitalbut no one was injured, Jose Abaca, a doctor at the hospital, said.

Students launched the first protest againstMichelle Bachelet, the president, last year by demanding free student access topublic transport.