KABUL- Afghan police fired into the air on Sunday to break up a protest by thousands of people who had gathered in the capital, Kabul, to protest against what they said was the desecration of a copy of the Koran by foreign troops.

Protesters, claiming foreign forces had burned a copy of Islam's holiest book during a raid in Maidan Wardak province last week, blocked traffic in Kabul for more than an hour.

A spokeswoman for U.S. and NATO-led forces in Afghanistan said none of their troops were involved in the incident and blamed the Taliban for spreading a false rumor that a copy of the Koran had been burned.

More than 100,000 foreign troops are battling a resurgent Taliban in Afghanistan, where violence this year reached its highest level since the austere Islamists were ousted by U.S.-backed Afghan forces in 2001.

Thick plumes of smoke rose above the crowd as protesters set fire to a large effigy of what they said was U.S. President Barack Obama.

Death to America. Down with Israel, chanted one man at the rally, which was organized mainly by university students.

Others threw stones and clashed with police but no casualties were reported.

No to democracy. We just want Islam, said one banner carried by protesters, many of whom shook their fists in the air.

Captain Elizabeth Mathias, a media officer for U.S. and NATO-led forces in Afghanistan, said the Taliban were trying to undermine foreign troops by spreading the rumor.

We did not burn a Koran ... It is unfortunate that the protesters believe a Taliban rumor, Mathias said, adding an investigation had been carried out.

The Taliban could not be reached immediately for comment.

Security is already being tightened across Afghanistan as the country braces for a run-off vote in its presidential poll on November 7 after the first round in August was tainted by allegations of widespread fraud.

The Taliban on Saturday vowed to disrupt the poll and urged Afghans to boycott the vote, as they had done before the August 20 first round.

Afghanistan has seen protests in the past over similar incidents, as well as over cartoons depicting the Prophet Mohammad in a Danish newspaper in 2006.

(Additional reporting and writing by Sayed Salahuddin; Editing by Paul Tait)