Afghan riot police and soldiers were on guard across Kabul and other cities in a bid to stop a fourth day of violent protests, with authorities worried about incendiary Friday mosque sermons over the burning of Korans at a NATO base.

Friday is a holy day and the official weekly holiday and large crowds were expected at major mosques in the capital, with police in armed pick-up trucks guarding surrounding streets and buildings.

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

Police are fully prepared to respond to situations, Sediqqi told Reuters.

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

Afghanistan wants NATO to put those responsible on public trial. 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.

In central Kabul, elite anti-riot officers in protective jackets and helmets secured intersections after complaints that security force numbers had been insufficient in protests so far that have left 11 people dead, including two American soldiers.

Troops and intelligence officials stood by in support, although 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.

Demonstrations in the last three days drew thousands of angry Afghans to the streets, chanting Death to America!, smashing shops and windows.

A large protest has already been planned for the eastern city of Jalalabad, where violent demonstrations have taken place in the last few days.

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

(Reporting by Rob Taylor; Editing by Nick Macfie)