Polls in Afghanistan's presidential elections have officially closed after a day of voting which saw sporadic violence.

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Nearly 6,500 poll centres opened at 7am (3.30am Irish time) but many reported a slow start to voting.

'Despite some initial attacks in the early morning, voters have demonstrated their determination to participate,' said Aleem Siddique, spokesman for the UN mission in Kabul.

He said reports from the field suggested robust turnout in the north and east, including among women. While turnout was slow this morning in the more violent south, many voted late as attacks tapered off.

A commission official said turnout could reach 50%, but other observers have predicted a worse turnout than for the first presidential vote in 2004.

There were reports of violence in a number of Afghan towns as polls opened this morning. Shops and businesses were closed and round-the-clock squads of extra police checked the few cars present on the streets in Kabul.

In the northern town of Baghlan, Taliban militants launched a multi-pronged assault that sparked heavy clashes, killing up to 22 militants and preventing voting, government officials said.

Provincial officials confirmed that scattered rockets hit the cities of Kandahar, Kunduz and Ghazni, and there were reports of rocket strikes in other towns.

The UN spokesman said there had also been a roadside bomb attack in Helmand province and some polling stations had been attacked in Khost.

Fragile democracy

President Hamid Karzai was one of the first to vote in an election seen as a tough test of his own mandate and his nation's fragile democracy.

He cast his ballot under tight security in a polling station at a high school near his presidential palace in Kabul, telling reporters he hoped for an outright majority in a single round.

'One round will be in the interest of the nation,' he said.

Mr Karzai faces an unexpectedly strong challenge from his former foreign minister, Abdullah Abdullah.

Official preliminary results are not expected for at least two weeks.

The election is also a test for US President Barack Obama, who has ordered a massive troop build-up this year as part of a strategy to reverse Taliban gains in the region.