FACTBOX: Key facts and figures about Afghan election

KABUL (Reuters) - Afghanistan went to the polls on Thursday for presidential and provincial elections against a backdrop of increased violence by the Taliban.

President Hamid Karzai is favorite to win, but unless he secures more than 50 percent of the vote in the first round he faces a run-off against the second-placed candidate. There are 40 challengers on the ballot paper but about 10 have withdrawn.

Here are some facts and figures about the election.

WHO IS REGISTERED AND WHERE WILL THEY VOTE?

The Afghan Independent Election Commission (IEC) estimates there are around 15 million eligible voters out of a population of around 33 million. About 4.5 million new voters registered this year, but those from previous years are still valid.

The electoral commission has sought clearance to open nearly 7,000 voting stations. IEC officials said on Monday about 440 stations would be closed because of security considerations.

WHO IS RUNNING?

There are now 31 candidates in the presidential race out of 41 originally registered, including two women.

Polls suggest President Hamid Karzai is in the lead, although not by enough to avoid a second round against former foreign minister Abdullah Abdullah. Former planning minister Ramazan Bashardost and former finance minister Ashraf Ghani are the only other candidates expected to get more than 2 percent of the vote.

Elections for provincial councils are also being held simultaneously, with more than 3,000 candidates vying for seats, about 10 percent of them women.

WHEN ARE RESULTS BE ANNOUNCED?

Preliminary results will be announced on September 3 and final results two weeks later. If no candidate wins more than 50 percent of all votes in the first round, the run off will be held between the top two, provisionally set for October 1.

WHO IS RESPONSIBLE FOR SECURITY?

Taliban insurgents have vowed to disrupt the elections and warned Afghans not to vote. There are more than 100,000 Western troops in the country, including 63,000 Americans, who will be operating mostly outside towns and villages on election day. Afghanistan's own forces of about 90,000 soldiers and 90,000 police are responsible for security in populated areas.

WHAT ROLE DOES THE INTERNATIONAL COMMUNITY PLAY?

To a greater degree than ever, the election is being run by Afghanistan's own election commission, with donors giving some $220 million to pay for it. Several international groups have sent monitors, with the European Union fielding a large mission.

WHAT ARE SOME OF THE CHALLENGES?

* Afghanistan is one of the poorest countries in the world and has suffered three decades of war. Only about a third of Afghans are literate.

* In some areas roads are so bad that ballot boxes have to be delivered by helicopters and donkeys.

* Taliban militants, stronger than at any time since they were driven from power eight years ago, have vowed to disrupt the election.

* Long distances, weak institutions and poor security may make it difficult to prevent fraud and abuse. (For more Reuters coverage of Afghanistan and Pakistan, see: here)

(Reporting by Hamid Shalizi; Editing by Paul Tait)