Hundreds of demonstrators marched towards the palace of Afghan President Hamid Karzai in Kabul on Friday, as a fourth day of protests erupted over the burning of Korans at a NATO base, with riot police and soldiers on high alert.

One protester was wounded by police gunfire, Reuters witnesses said.

Police fired into the air to try to disperse the crowd of men throwing stones and chanting Death to America! and Long live Islam! after they left the city's Blue Mosque following Friday prayers.

Friday is a holy day and the official weekly holiday in Afghanistan and mosques in the capital drew large crowds, with police in pick-up trucks posted on nearby streets.

Crowds of about 700 protesters also gathered in the eastern city of Jalalabad and the volatile southeastern province of Ghazni, where people chanted We will defend the Koran!, Reuters witnesses said.

U.S. President Barack Obama sent a letter to Karzai apologising for the unintentional burning of the Korans at NATO's main Bagram air base, north of Kabul, after Afghan labourers found charred copies while collecting rubbish.

Muslims consider the Koran to be the literal word of God and treat each copy with deep reverence. Desecration is considered one of the worst forms of blasphemy.

Afghanistan wants NATO to put those responsible on public trial.

These apologies are fake. The world should know that America is against Islam, Iranian cleric Ahmad Khatami said in a speech broadcast live on state radio.

It (the Koran burning) was not a mistake. It was an intentional move, done on purpose.

In central Kabul, elite anti-riot officers in protective jackets and helmets secured intersections after complaints that security force numbers had been insufficient to deal with the protests in which 11 people, including two U.S. soldiers, have been killed.

Although peaceful demonstrations are the right of people, we strongly urge our countrymen to fully avoid turning them into violent ones, said Interior Ministry spokesman Sediq Sediqqi.

Most Westerners have been confined to their heavily fortified compounds, including at the sprawling U.S. embassy complex and other diplomatic missions.

The embassy, in a message on the microblogging site Twitter, urged U.S. citizens to please be safe out there and expanded movement restrictions to relatively peaceful northern provinces, where large demonstrations also occurred on Thursday, including the attempted storming of a Norwegian military base.

The Taliban urged Afghan security forces on Thursday to turn their guns on the foreign infidel invaders and repeatedly urged Afghans to kill, beat and capture NATO soldiers.

(Writing by Amie Ferris-Rotman; Editing by Nick Macfie)