Two bombs have exploded outside a hotel in western Kabul where presidential candidate Abdullah Abdullah was holding a rally.
Abdullah, a former Afghan foreigh minister, said his car was hit in the blast but that he was not injured.
A spokesman for the Afghan Interior Ministry said four civilians were killed in the blast.
At another rally shortly afterward, Abdullah said he had been leaving a gathering of the People's Islamic Unity Party when his car was struck by a roadside bomb and damaged.
No one has so far claimed responsibility for the attack.
Abdullah is considered to be the front-runner in the second-round presidential vote, scheduled for June 14. He faces former Finance Minister Ashraf Ghani.
Both Ghani and Afghan President Hamid Karzai condemned the attack, both calling it an act of the "enemies of Afghanistan" to disrupt the democratic process.
In a separate incident, insurgents targeted the head of Abdullah’s campaign team in Faryab Province on June 6.
General Abdul Malek, who leads the team, told RFE/RL that he and his brother escaped the attack but that a bodyguard was killed.
With reporting by Reuters and AFP