Image copyright AFP Image caption An election poster for Abdul Rasul Sayyaf in Kabul

The campaign for Afghanistan's presidential election has been formally launched, with rallies for key candidates in the capital, Kabul.

The election is scheduled to take place on 5 April.

There are 11 candidates to replace Hamid Karzai, who cannot stand again.

Security is the major issue, with the Taliban threatening to target the campaign. Two workers for candidate Abdullah Abdullah were shot dead in the western city of Herat on Saturday.

Security fears

Thousands of people attended rallies in halls in Kabul on Sunday as the campaign started, with posters now dotted around the city.

All the candidates have been given armoured vehicles and security guards.

Academic Ashraf Ghani, who is running with former Uzbek warlord Gen Abdul Rashid Dostum and former justice minister Sarwar Danish as vice-presidents, promised at his rally to bring reforms.

Image copyright Reuters Image caption Supporters of Abdullah Abdullah attend his rally in Kabul

Image copyright Reuters Image caption Voters also gathered to hear Ashraf Ghani in Kabul

Mr Abdullah also held a rally.

He said the world should not be frightened of Afghanistan, nor should Afghans be afraid of the world.

One of those who attended, Arefa Alizada, told Agence France-Presse news agency he was concerned about security, referring to the deaths of the two campaign workers.

"If it worsens, I and many other people won't be able to vote," he said.

One of the main campaign issues will be the presence of US forces beyond the end of the year - when Western forces are scheduled to leave.

A deal with the US was reached to retain thousands to train and mentor local security forces.

However, Mr Karzai has stalled on signing the agreement himself, suggesting his successor would complete the negotiations.

Image copyright AFP Image caption Security will be the main issue both during the election campaign and after it

The Taliban has rejected the election and attacks in the capital are already on the increase.

The UN condemned Saturday's attack in Herat, saying: "This cowardly action constitutes a violent intimidation of electoral candidates and their supporters, and cannot be tolerated."

The last election, in 2009, was also dogged by allegations of vote rigging and intimidation.

Mr Abdullah pulled out of a run-off vote, handing Mr Karzai another term.

The BBC's David Loyn in Kabul says a clear winner in the first round is again unlikely, meaning a result may not be known until the summer.

Karzai's brother, Qayum, and key ally Zalmai Rassoul, could be key challengers.

Another candidate, religious scholar Abdul Rasul Sayyaf, has a jihadist background that may concern the West.

Ethnic backgrounds will be a major issue in the election. The CIA World Factbook provides estimates for some of Afghanistan's key ethnic groups: Pashtun (42%), Tajik (27%), Hazara (9%), and Uzbek (9%).