SEOUL, South Korea — Thousands of South Korean students rallying Sunday against the U.S. military's five-decade presence clashed with police after trying to enter the American base, and at least 12 people were injured and more than 20 were arrested. Demonstrators marched through Seoul before attempting to enter the main Yongsan U.S. military base in the city center. They called for the withdrawal of the 32,500 U.S. troops stationed in South Korea, a legacy of the 1950-53 Korean War.Demonstrators also gathered near the U.S. Embassy in downtown Seoul demanding talks with the ambassador. Television pictures showed masked protesters repeatedly charging helmeted riot police, who wielded truncheons and carried shields. At one point, students lay in the street outside the base, chanting and clapping. South Korea's Yonhap news agency said the protest was the largest of its kind in recent years. Such rallies were common in the 1980s and 1990s but have lately given way to peaceful candlelight vigils. There were no exact figures on the number of protesters, although Yonhap said they numbered in the thousands.