Several people were wounded on Wednesday, witnesses said, when shots were fired as hundreds of angry Afghans gathered in a second day of violent clashes after copies of the Koran, Islam's holy book, were burned at NATO's main base in Afghanistan.

Reuters witnesses said the shots were fired into demonstrators when they charged at police lines and smashed car windows. It appeared police had fired the shots but there was no immediate confirmation from Afghan security forces.

Protesters shouted Death to America! and Death to (President Hamid) Karzai in a large demonstration on the outskirts of the Afghan capital, Kabul. A second rally had begun in another area of the city, Reuters witnesses said.

When the Americans insult us to this degree, we will join the insurgents, said Ajmal, an 18-year-old protester in Kabul, where dozens of protesters charged through police barriers.

Muslims consider the Koran the literal word of God and treat each book with deep reverence.

Winning the hearts and minds of Afghans is critical to efforts to defeat the Taliban. Similar incidents in the past have caused deep divisions and resentment among Afghans towards the tens of thousands of foreign troops in Afghanistan.

Critics say Western troops often fail to grasp the country's religious and cultural sensitivities.

Separate protests were also underway in Jalalabad in the east, where demonstrators praised the leader of the Afghan Taliban, the secretive Mullah Mohammad Omar, screaming Long live Mullah Omar!, Reuters witnesses said.

Afghan media said demonstrations had also erupted in the western city of Herat, usually one of the more stable areas in a country devastated by three decades of conflict.

In Kabul, protesters smashed car windows while police fired water cannon in a bid to disperse the angry crowd which had blocked a major road.

The U.S. government and the U.S. commander of NATO-led forces in Afghanistan apologised on Tuesday after Afghan labourers found charred copies of the Koran while collecting rubbish at the sprawling Bagram Airbase about an hour's drive north of Kabul.

Demonstrations by as many as 2,000 people broke out as word of the find spread.

U.S. Defense Secretary Leon Panetta issued an apology for inappropriate treatment of copies of the Koran at the base to try to contain fury over the incident -- a public relations disaster for Washington as it tries to pacify the country ahead of the withdrawal of foreign combat troops by the end of 2014.

Seven foreign UN workers were killed during protests that raged across Afghanistan for three days in April 2011 after a U.S. pastor burned a Koran in Florida.