KABUL, Afghanistan -- An angry protest erupted outside a sprawling U.S.-run military installation Tuesday after base personnel allegedly improperly disposed of religious materials, including Muslim holy books.

Gen. John Allen, the U.S. Marine who commands Western forces in Afghanistan, apologized and promised that the incident at Bagram Air Field, north of Kabul, would be investigated.

Protests over desecration of the Quran have sparked serious violence in the past. One of the worst such incidents occurred last April, when a furious mob overran a United Nations headquarters in the city of Mazar-e-Sharif, killing four Nepalese guards and three foreign U.N. workers, after a fundamentalist preacher in Florida burned the holy book.

In Tuesday's protest, four people were injured when guards fired on the crowd, according to Afghan officials in Parwan, where the Bagram base is located. A spokesman for NATO's International Security Assistance Force declined to say whether Western troops were involved in any crowd-control actions during the demonstration.

Abdul Aziz, the Bagram district police chief, said the incident began before dawn when Afghans working as laborers inside the base found several books, including Qurans, being burned and brought them outside.

"This provoked the local people's emotions, and they started a protest," Aziz said.

It was not known whether the materials had been inadvertently placed in the base's trash collection. Most refuse is disposed of by burning.

Aziz said "hundreds" had taken part in the demonstration. The ISAF did not provide a crowd estimate.

The commanding general's statement, issued hours after the incident, was unusually rapid and uncommonly contrite.

Addressing the "noble people of Afghanistan," Allen said he had been given a report overnight indicating that Western military personnel at Bagram had "improperly disposed of a large number of Islamic religious materials," including Qurans.

"When we learned of these actions, we immediately intervened and stopped them," he said, adding that the recovered materials would be turned over to religious authorities. He thanked the local people who had brought the matter to light.

"We are thoroughly investigating the incident, and we are taking steps to ensure this does not ever happen again," Allen said. "I assure you ... I promise you ... this was NOT intentional in any way."