At least four people have died and many were hurt after police fired shots to disperse anti-US protests in the Afghan city of Jalalabad, officials say.
Protests in Jalalabad turned violent
Hundreds rioted over reports that interrogators at America's Guantanamo Bay prison had flushed at least one copy of the Koran down a toilet.
The city is now said to be calm after widespread damage to property. All but essential UN staff are being withdrawn.
US authorities have said they are investigating the Koran allegations.
"Obviously the destruction of any kind of holy book... is something that is reprehensible and not in keeping with US policies and practices," state department spokesman Tom Casey said.
President Hamid Karzai said the violence showed the inability of Afghan authorities to handle such protests.
Speaking at Nato headquarters in Brussels, he said his country would need international assistance "for many, many years to come".
Buildings burned down
Afghan National Army soldiers were deployed on the streets of Jalalabad, which lies 130km (80 miles) east of Kabul and is close to the Pakistani border, to try to bring order.
A US military spokesman said US units were not deployed.
The Afghan interior ministry says four people were killed, and more than 70 injured in a second day of unrest.
"Police opened fire in the air to control the mob, and some people were injured," Jalalabad police chief, Abdul Rehman, told the AFP news agency.
Two United Nations guest houses were attacked, as were shops and government buildings, and the offices of two international aid groups were destroyed.
Many foreign aid agency personnel are being pulled out too.
Pakistani ambassador to Afghanistan, Rustam Shah Mohmand, told the BBC the Pakistani consul's house had also been burned down and two cars torched.
The protesters chanted "Death to America" and smashed car windows and damaged shops.
Smoke could be seen rising from various points in the city.
One international aid worker in Jalalabad told the BBC there were groups of people running along the streets, reportedly looking for foreigners and anyone working for non-governmental organisations.
Another eyewitness said: "They were shouting anti-Karzai and anti-Bush slogans. Angry demonstrators carrying sticks and stones clashed with police forces in the city centre."
There were also reports of anti-US protests in the south-eastern city of Khost, and in Laghman province, where a demonstration was dispersed peacefully by police.
BBC correspondents say many Afghans are angry at what they regard as heavy-handed tactics used by American forces in their country.
Aid agencies have also become a focus of resentment for many Afghans who feel they have yet to see the benefit of the billions of dollars of international aid money pumped into the country.
Reports of abuse
The unrest follows a recent report in the American magazine, Newsweek, that interrogators at Guantanamo Bay had desecrated the Koran in order to put pressure on Muslim prisoners.
Investigators probing reported abuses at Guantanamo Bay found that interrogators "had placed Korans on toilets, and in at least one case flushed a holy book down the toilet," Newsweek said.
Hundreds of inmates are still being held at Guantanamo Bay
Former Guantanamo inmates told the BBC Urdu service earlier this month that some Arab prisoners had still not spoken to their interrogators after three years to protest at the desecration of the Koran by guards at the camp.
On Sunday, the Pakistani government said it was "deeply dismayed" over the reports about the Koran.
Islamist parties there have called for a nationwide strike on Friday.
Both Pakistan and Afghanistan are close allies of the US in its war against terror.
Insulting the Koran or the Prophet Muhammad is regarded as blasphemy and punishable by death in both Pakistan and Afghanistan.
The US is holding about 520 inmates at Guantanamo Bay, many of them al-Qaeda and Taleban suspects captured in Pakistan and Afghanistan following the 11 September 2001 attacks in the US and subsequent US-led invasion of Afghanistan.