Hundreds of demonstrators marched towards the palace of Afghan President Hamid Karzai in the capital, Kabul, on Friday, chanting "Death to America!," as a fourth day of protests erupted over the burning of Korans at a NATO base.

Protests of about 700 people were also going on in the eastern city of Jalalabad and the volatile southeastern province of Ghazni, Reuters reported.

Over a dozen of people were killed, including two NATO soldiers, in the continuous violence across Afghanistan, after the burning of the Koran.

Many others were injured in the protests, while armed men also attacked at least two military installations.

U.S. President Barack Obama has sent his Afghan counterpart Hamid Karzai a letter of apology for the burnings.