A suicide bomber rammed his car into an Afghan army vehicle providing security for a compound where Afghanistan's political and tribal elites are due to gather next week to debate a security pact with the US.
Today's attack took place just hours after President Hamid Karzai called on the Taliban to take part in the Loya Jirga assembly.
The assembly convenes next Thursday to decide whether to allow some US troops to stay in Afghanistan after 2014.
At least six people were killed and 22 injured in the blast that the Taliban claim they were behind.
The casualties were a mix of soldiers and civilians.
The blast occurred shortly after 3 pm, fewer than 100m from a huge tent where more than 2,000 prominent Afghans are due to gather.
A Kabul police spokesman, Hashmat Stanekzai, said earlier there were at least 15 dead and wounded as a result of the attack, but could not immediately provide more detail.