Protesters wielding bamboo sticks fought on Thursday with police armed with batons and metal shields at several spots in the area, about 70km south of Seoul on Thursday.
Scores of protesters suffered injuries in the clash and they said the number will grow.
The confrontation has been brewing for months since about 100 farmers refused to vacate the area where South Korea and the United States agreed two years ago to move the main US military base now in Seoul and several others throughout the country.
The number of protesters, many of whom are anti-US military activists, unionists and students, grew sharply on Wednesday night in expectations the government will send in thousands of police and military engineers to evict the area.
Last-ditch talks between the government and local residents for a compromise broke down on Monday. Local residents, mostly elderly farmers, and protesters have said no amount of compensation can justify the move.
About 30,000 US troops are stationed in the country in combined defence with the South Korean military against possible aggression by communist North Korea.
South and North Korea are technically at war under a truce that ended the 1950-53 Korean war.
The South Korean government has said further delay in the base relocation could cause diplomatic friction with the United States.