The US commander in Afghanistan has apologised after claims American troops set fire to copies of the Koran at the main military base.
Allegations that Afghan labourers had found charred remains of copies of the Koran as they collected rubbish from Bagram airbase led to thousands of protesters attacking the site.
Angry demonstrators fired slingshots and hurled petrol bombs at the base north of Kabul, it was reported.
Guards at the base apparently responded by firing rubber bullets from a watchtower.
Sidiq Siddiqi, an interior ministry spokesman, said reinforcements were sent to prevent the demonstration from spiralling out of control.
US commander, General John Allen, apologised over reports US troops had "improperly disposed of a large number of Islamic religious materials which included Korans".
He said he had ordered a full inquiry into the alleged incident.
"When we learned of these actions, we immediately intervened and stopped them," he said.
"The materials recovered will be properly handled by appropriate religious authorities.
"We are thoroughly investigating the incident and we are taking steps to ensure this does not ever happen again.
"I assure you - I promise you - this was not intentional in any way."
He added: "I offer my sincere apologies for any offence this may have caused, to the president of Afghanistan, the government of the Islamic Republic of Afghanistan, and most importantly, to the noble people of Afghanistan."
US defence secretary Leon Panetta also apologised for the "inappropriate treatment" of copies of the Koran and promised a swift investigation.
A separate protest by around 500 people erupted in the Pul-e-Charkhi district of Kabul, near another major Nato base.
Afghanistan is an extremely devout Islamic nation where an insult to the religion carries the death penalty.
Similar protests in the past have turned violent.
Ten people were killed and several injured during several days of unrest last April after the burning of a Koran by American pastor Terry Jones in Florida.