Afghans vote in a presidential election that marks the first democratic transfer of power since the fall of the Taliban in 2001.

The incumbent Hamid Karzai was among the early voters.

He's barred by the constitution from standing again after 12 years in power.

Eight candidates are battling it out to succeed him.

Of those, political observers say there are three front runners most likely to win.

Former foreign minister Abdullah Abdullah is one of them.

Zalmay Rassoul, also a former foreign minister, is another.

Ashraf Ghani, a former finance minister, is the third.

Voting is taking place amid tight security after Taliban threats to disrupt the polls through bombings and assassinations.