Chilean police arrested 323 people as demonstrations rocked the country's three largest cities, officials reported. In Santiago, troops fired on rock-throwing demonstrators in a slum district, and a 2-year-old boy was killed by a stray bullet, authorities and radio reports said. At least three other people were wounded as the government of President Augusto Pinochet attempted to control a day of protest called by an umbrella group of unions. Public transportation was disrupted as riot police used tear gas and water cannon to disperse protesters in downtown Santiago. The protests, called to demand wage increases, were the most serious this year against the military regime.