Afghan and US officials are investigating how many civilians died
Hundreds of people have taken to the streets in Farah in western Afghanistan in protest at the deaths of civilians in US air strikes earlier this week.
Shots were fired into the air and stones were thrown at government buildings. A number of people are reported to have been injured.
Witnesses said the crowd chanted "death to America, death to the invaders", and demanded US forces leave Afghanistan.
The protest comes as President Hamid Karzai continues his visit to the US.
The air strikes overshadowed a summit in Washington on Wednesday between President Barack Obama, Mr Karzai and Pakistani President Asif Ali Zardari.
US Secretary of State Hillary Clinton said she "deeply, deeply" regretted the deaths, adding that the US would work hard to avoid such "loss of innocent life".
A Pentagon official said so far it was not clear who was to blame for the death of the civilians.
"Initial American investigation shows that some of the deaths do not seem to be in concert with how a civilian would die from an air strike," the official said.
The New York Times quotes Pentagon officials as saying investigators were looking into the possibility that Taliban militants were responsible for the casualties.
Afghan officials say more than 100 civilians died in the attacks.
Deputy provincial governor Mohammad Younus Rasouli described the protests on Thursday as "violent".
"Police tried to disperse them but they started throwing stones at police, who fired into the air," he told AFP news agency.
One protester, Haji Nangyalai, 42, said he was demonstrating to "show our anger at the crimes committed by the American forces".
"We ask the Afghan government to force the American forces to leave Afghanistan. They kill more civilians than Taleban," he said.
Afghan police said more than 100 people had died, most of them civilians. One Afghan official said he had counted the bodies of 20 children.
The Red Cross has said that women and children were among dozens of dead.
President Karzai has long pleaded with Washington to minimise civilian deaths, saying such killings undermine the fight against the Taleban.
US President Barack Obama said after meeting his Afghan and Pakistani counterparts in Washington that the three leaders were united in the goal of defeating al-Qaeda.
He said he expected more setbacks and violence to come, but there was a lasting commitment to defeat the militants.
The BBC's Martin Patience in Kabul says President Karzai is now back in favour at the White House following months of fierce criticism from US officials.
But our correspondent says he will have to face growing anger over the civilian casualties when he returns home.
US Defense Secretary Robert Gates visited a US Marines base in southern Afghanistan on Thursday, and said there were no plans to deploy US troops in Pakistan.
The Pakistani army is currently engaged in fierce fighting against Taleban insurgents in the north-west of the country.