Thousands of workers entered the city after a three day march against fuel subsidy cuts. The protest targets a decree adopted in December by President Rodrigo Paz, ending long standing support for petrol and diesel.
Prices quickly doubled, pushing up transport and food costs. Many marchers fear higher inflation in a country already facing its worst economic crisis in decades, after selling imported fuel at a loss drained foreign currency reserves.
As the crowd reached the city centre, riot police blocked access to the main square using tear gas. Authorities said 12 people were detained, some accused of carrying dynamite.
Talks opened between union leaders and the government but were suspended without agreement. Protest organisers warn of road blockades if the decree remains in place.
