On Thursday, military engineers moved in and cordoned off two townships about 70 km (44 miles) south of Seoul, where South Korea and the United States have agreed to relocate the main US base now in Seoul and several others across the country.
Scores of protesters, police and several journalists were wounded as riot police cleared the way for the engineers, fighting pitched battles with protesters and eventually overpowering them.
Friday's fight started outside the townships as about 1500 anti-US protesters and unionists regrouped overnight and began marching into the fenced-off site.
Reuters’ photographer Lee Jae-won saw several people on both sides hurt in the fight.
What began as rowdy shoving and fisticuffs involving hundreds on both sides turned more violent as the day progressed. Some protesters broke through a newly erected fence and charged at unarmed soldiers with sticks, injuring scores.
"The enforcement of administrative order for the US base relocation was inevitable if we were to prevent large losses to our national interest diplomatically and economically that would be caused by delay"
Statement by the presidential office
The defence ministry said 11 soldiers suffered serious injuries and were airlifted by helicopter to a military hospital just south of Seoul. It was not known if any of the injuries were life-threatening, a defence official said.
Witnesses on the scene said fighting continued late into the night with soldiers, who are under orders to refrain from fighting back, defending themselves.
The confrontation that erupted on Thursday had been brewing for months as about 100 farmers refused to vacate the area.
Last ditch talks between the government and residents aimed at a compromise broke down on Monday. Local residents who remained, mostly elderly farmers, and protesters have said no amount of compensation would justify the move.
"Do not insult the residents who are fighting here," said Kim Ji-tae, who leads the farmers remaining and protesters in the area, in an open letter to President Roh Moo-hyun on Friday.
"As we said a number of times, we are not interested in compensation. What we want is to continue living here."
Yoon Kwang-ung, the defence minister, said the base relocation, which had been agreed to by Seoul and Washington in 2004 and authorised by South Korea's parliament, could no longer wait.
About 30,000 US troops are
stationed in South Korea
He said the delay was caused by opponents of US military presence in the country who had taken advantage of the farmers.
The presidential office in Seoul said in a statement it regretted the injuries suffered by the protesters and police.
"The enforcement of administrative order for the US base relocation was inevitable if we were to prevent large losses to our national interest diplomatically and economically that would be caused by delay," the statement said.
About 30,000 US troops are stationed in the country alongside the South Korean military to guard against possible aggression by communist North Korea.
South and North Korea remain technically at war under a truce that ended the 1950-53 Korean War.