At least four people were killed when police opened fire on hundreds of angry protesters during a second day of demonstrations aimed at NATO troops who improperly disposed of copies of the Quran.

Afghan officials say dozens were injured in the shootings which occurred in the capital, Kabul, Parwan province, and the eastern city of Jalalabad.

Crowds in the capital, shouted chants of “Death to America” while hurling stones at officers, setting fire to cars and buildings and blocking some main roads.

The U.S. Embassy in Kabul says it has placed its staff on lockdown and suspended all travel.

The commander of the international coalition, U.S. General John Allen, on Tuesday issued an apology and ordered an investigation into a report that coalition forces “improperly disposed” of a large number of Islamic religious texts, including the Quran.

“I assure you…I promise you…this was not intentional in any way. I offer my sincere apologies for any offense 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.”

The circumstances surrounding the alleged desecration are unclear. Unconfirmed reports suggest that NATO troops stationed at Bagram Air Base outside of Kabul attempted to dispose of a load of Qurans by setting them on fire, but were stopped by Afghan employees there.

Afghan protests against the destruction of the Muslim holy book have turned deadly in recent years. In April 2011, about 20 people were killed during several days of protests across Afghanistan after little-known U.S. pastor Terry Jones burned a Quran at his small Florida church.