Candidates to replace Afghan President Hamid Karzai are facing off in the first televised debate of a crucial election campaign ahead of the planned withdrawal of NATO combat troops.

A successful election would mark Afghanistan's first successful democratic transfer of power. It will be a crucial test of whether the country can ensure a stable transition after years of war and while facing a Taliban insurgency that has vowed it will attempt to disrupt the election.

Reflecting the fears of violence, security was tight for Tuesday's debate with five of 11 candidates: Abdullah Abdullah, runner-up in the 2009 election; former foreign minister Zalmai Rassoul; ex-finance minister Ashraf Ghani; ex-defense minister Abdul Rahim Wardak and Karzai's brother, Qayyum Karzai.

Hamid Karzai is ineligible to run.