Presidential front-runner Abdullah Abdullah has survived an assassination attempt on his convoy in western Kabul.
It is reported that bodyguards were injured but Abdullah was unharmed when two bomb blasts hit his presidential rally in the Afghan capital.
Police claimed that there were several casualties when the devices detonated but the death toll could not be immediately verified.
Abdullah's election rival Ashraf Ghani condemned the attack. He tweeted: "I have just been informed about a terrorist attack on Dr sahib Abdullah's campaign vehicle, strongly condemn this."
"This is the act of the enemies of Afghanistan to disrupt the democratic process in the country," he continued.
No group has yet claimed responsibility for the blast but Taliban militants have threatened to strike in the Afghan capital ahead of the second round of a presidential election run-off on June 14.
Last month, Taliban fighters beheaded eight policemen in the southern province of Zabul and killed at least eight others in Badakhshan.