South Korean protesters opposed to a free trade deal with the United States fought pitched battles with riot police on Sunday, as thousands of demonstrators jammed the streets of downtown Seoul. Police, using truncheons and water cannon, detained scores of farmers and student activists, according to Reuters witnesses. Organisers estimated 50,000 people took part in the protest, while police put the number of demonstrators at 20,000. At one location, police beat male and female college students with batons, forcing protesters to the ground and then kicking several of them into submission before dragging them away, one Reuters witness said. South Korea and the United States struck a sweeping bilateral free trade deal in April that studies say could boost their $73 billion-a-year annual trade by about a further $20 billion.