The bloodshed in the capital comes amid an ongoing surge in attacks across Afghanistan, where some 1,500 people were killed or injured last month alone.
A huge plume of black smoke was seen rising over western Kabul following a massive explosion at the entrance to a police compound around 9:00 am (0430 GMT).
The Afghan interior ministry said the explosion was caused by a car bomb, but the Taliban, which claimed the attack, said it was a much larger truck bomb. An Afghan security official said it was a truck bomb.
The attack came after the Taliban warned Afghans on Tuesday to keep away from public gatherings ahead of the presidential election planned for September 28.