DISSIDENTS, CATHOLIC PRIESTS, professors and students kept up protests in Seoul, South Korea, Thursday against the death of a student caused by riot squads and urged the government to stop using excessive public power to maintain its rule. . . . HUNDREDS OF HUNGARIAN Catholics, many crying and singing national songs, converged on a church in Mariazell, Austria, Thursday to honor Cardinal Jozsef Mindszenty before his remains are returned to Hungary. . . . LEFTIST REBELS FIRED a rocket into a power company office in Athens, Greece, before dawn Thursday, causing damage but no injuries.