A suicide bomber and a roadside bomb have struck the convoy of Afghanistan's presidential candidate Abdullah Abdullah as it left a campaign event in the capital Kabul, the Interior Ministry said.
Abdullah said he was unhurt in Friday's attack, but at least six people were killed and 22 others wounded, according to the ministry.
"A few minutes ago, when we left a campaign rally our convoy was hit by a mine," Abdullah told supporters shortly after the attack, in quotes broadcast on Afghan television.
He said several of his security guards had been wounded in the attack, which occurred soon after Abdullah left a rally at the city's Ariana hotel.
The assassination attempt came ahead of a second-round presidential election on June 14, which Taliban fighters have threatened to disrupt.
No group has so far claimed responsibility for Friday's attack, the AFP news agency reported.
Afghanistan is in the middle of elections to choose a successor to President Hamid Karzai, who has ruled since the fall of the country's Taliban government in 2001.
|Inside Story: What does a US drawdown mean for Afghanistan?
Abdullah fell short of the 50 percent threshold needed for an outright victory in the April first round and will face former World Bank economist Ashraf Ghani in the runoff.
Ghani and Karzai condemned the attempt on Abdullah's life, with Ghani calling it "the act of the enemies of Afghanistan to disrupt the democratic process in the country".
The UN Security denounced the attack and "attempts to disrupt elections by targeting election personnel, candidates or infrastructure".
The United States also condemned the assassination bid, saying Afghanistan deserved a democratic future.
US President Barack Obama recently outlined the US strategy to end the war in Afghanistan, saying the US presence in the country would be scaled back to around 9,800 troops by the start of 2015.
Those forces would be halved by the end of 2015 before eventually being reduced to a normal embassy presence with a security assistance component by the end of 2016.
The drawdown relies on Afghanistan signing a long-delayed Bilateral Security Agreement laying out the terms of the US military presence in the country after this year.
The outgoing Karzai refuses to sign the pact, but both Afghan presidential candidates have vowed to sign it if elected.