The Afghan government imposed an overnight curfew across Kabul this evening after a day of anti-America street riots in which at least 13 people were killed and dozens more injured.

Officers and cars were set alight by angry mobs who spent several hours rampaging through the capital in its worst day of unrest since the US-led invasion four years ago.

Commentators said that the outburst of violence could signal a turning point in relations between the local population and the American forces which until now, although tense, have been peaceable.

The riots were triggered when a US military convoy of Humvees and armoured lorries attempted to flee after three people died in a traffic pile-up on the outskirts of the city at rush hour early this morning.

The driver of a US military cargo truck lost control of his vehicle and plowed into at least 12 cars at a busy intersection. There were reports that US troops fired into the crowd as angry drivers began pelting their vehicles with stones.

Afghan police and troops were called in to rescue the convoy and more shots were fired: there are conflicting reports over who began the exchange of fire but at least one more person was killed.

The bloodshed sparked a rapid escalation in the violence. Soon, almost 2,000 people were attempting to march into the city centre armoured with sticks and swords intent on demonstrating outside the US embassy and the heavily fortified palace.

They were met by ranks of Afghan security forces triggering clashes on dusty roads through Kabul. Shops and officers were looted. Police cars and a small police station were set on fire. The offices of Care International were ransacked.

Eyewitnesses told AFP that they had seen another man shot dead outside the Serena Hotel, a complex popular with Westerners and close to the presidential palace.

Bursts of automatic gunfire were also heard outside the US Embassy. Officials were moved to secure bunkers within the compound.

As the crowds dispersed, still chanting "Death to Karzai... Death to America", smoke hung over the city centre. There were reports of sporadic gunfire throughout the afternoon.

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