The incident has seriously hurt our national pride
On Wednesday US President George Bush sent a message of regret for the deaths of the girls, who were crushed by a US military vehicle in June.
But critics said the apology was not enough to stem the anger in South Korea, which hosts 37,000 US soldiers.
Last week a US military court cleared Sergeant Fernando Nino and Sergeant Mark Walker of negligent homicide, but anti-US protesters said the verdict was a sham.
"I don't want to give a drop of water to Americans," said Lee Chang-yong, the owner of Zeno restaurant which serves spaghetti and barbecued ribs.
"The incident has seriously hurt our national pride."
A few anti-US signs have also appeared near Seoul's Hongik University, an entertainment district, but the signs are not widespread.
Most South Koreans support or tolerate the presence of US troops, who are there as a deterrent against North Korea.
But there was public anger that the US refused to allow the two soldiers to be tried in a Korean court.
Some protests have turned violent, with Molotov cocktails being thrown into military bases.
On Thursday two firebombs were thrown over the wall of Camp Page in the eastern city of Chuncheon.
On Tuesday about 50 people were arrested following an attack at a major military base north of Seoul.
Security has been stepped up around the US embassy and other US buildings around the country, and 14,000 soldiers in the 2nd Infantry Division - which was home to the acquitted men - are observing a 2200 curfew.
General Leon LaPorte, commander of the US troops in South Korea, said he would do what was necessary to ensure the safety of soldiers and their families.
"If required, I will have to initiate some actions so we can ensure that our service members and family members are not exposed to violence," he said on Wednesday.
Mr Bush's apology was conveyed though US Ambassador Thomas Hubbard, who said the president wanted to express "sadness and regret" over the deaths of the two girls.
The two acquitted sergeants have now left South Korea.