(CNN) Protesters and police clashed once again in the Chilean capital of Santiago on Thursday, as authorities extended a curfew for the sixth consecutive night. An apology from President Sebastian Pinera failed to temper escalating levels of violence on the streets, which have so far led to at least 18 deaths.

The United Nations human rights council plans to send a special mission to investigate allegations of rights abuses in the South American country amid a week of deadly clashes between protesters and police, UN human rights chief and former Chilean president Michelle Bachelet said Thursday.

Protests have paralyzed parts of the capital city for seven days in a row and Chilean police have responded to demonstrators by firing tear gas, rubber bullets, and water cannons.

In a national televised address Tuesday night, Pinera apologized for "decades" of accumulated problems and announced a new social and economic agenda in the wake of the deadly unrest.

"It's true -- problems have not occurred in recent days. They have been accumulating for decades," he said of the South American country of 18 million people.

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