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The demonstrators themselves sid they were angry about the burning.
They had been told that hundreds were burned.

A large and angry crowd surged through the streets of Kandahar, chanting "They have insulted our Koran" and "Death to America", the BBC's Paul Wood reports from Kabul.

The authorities in both Kandahar and Mazar-e Sharif blamed the Taliban for the attacks. However, the Taliban has rejected the accusation.
Mazar-e Sharif is due to be one of the first places transferred from Nato security control in the summer, but on this occasion the Afghan police were unable to protect the UN.
"The Taliban had nothing to do with this, it was a pure act of responsible Muslims," spokesman Zabiullah Mujahid told the Reuters news agency by phone from an undisclosed location.

"The foreigners brought the wrath of the Afghans on themselves by burning the Koran," he said.