Hundreds of protesters marched through the streets of Kabul on Monday after a US military vehicle killed at least one Afghan. Witnesses alleged that US and Afghan forces opened fire after the incident, killing at least eight people.
The violence had calmed by the early evening, although sporadic clashes were reported near the scene of the fatal crash.
The Afghan government imposed a six-hour curfew as of 10pm local time following the clashes.
Karzai later appeared on television to condemn the violence, saying it was caused by people looking to capitalise on the accident and destroy the country.
"My wish for my countrymen is for them to act seriously whenever they confront such elements and not allow them to destroy our home again," he said.
Karzai did not specify who the agitators were, although the country is currently battling a resurgence in attacks from fighter's loyal to the country's former Taliban government.
Witnesses said US and Afghan
forces opened fire on protesters
The unrest began in the morning after three US Humvees en route from the city of Bagram ran into a rush-hour traffic jam, hitting several civilian cars, witnesses said.
Captain Frank Pasqual, a US military spokesman in Dubai, told Aljazeera that one person was killed and six injured in the incident, which he blamed on a mechanical failure.
However, a Kabul police chief, Sher Shah Usafi, said at least three people were killed and 16 wounded in the crash.
Witnesses said that US forces then opened fire on protesters who had gathered in anger over the incident. Afghan forces who arrived to help the US troops also opened fire, they said.
Hospital sources said at least eight people had died, although tolls vary.
One wounded protester told Aljazeera: "Karzai said they came to help us, now they are killing us."
An AFP photographer at the scene of the accident said US troops shot dead at least four people. He said two men were shot dead next to him and two other bodies were found after the burst of gunfire. Several were wounded.
Pasqual could not confirm if US forces had subsequently opened fire on protesters, although Colonel Thomas Collins, a coalition spokesman in Kabul, said that US personnel had fired over the protesters' heads.
The United States has 23,000 troops in Afghanistan while a Nato-led peacekeeping force has more than 9,000 troops in the country.
"Karzai said they (US troops) came to help us, now they are killing us"
Most Nato forces are stationed in Kabul and the more peaceful north and west. It is currently expanding its mission to the south.
A Taliban commander this week said the Taliban had regained control of the southern provinces in its fight to remove foreign forces from the country and overthrow the US-backed government.
The Taliban said on Monday that the American reaction against the crowds debunked the notion that the US occupation was to help the Afghan people.