The lunch time bomb blast on Tuesday occurred near the presidential palace and various ministries.
Yousuf Stanizai, a spokesman for the interior ministry, blamed "enemies of the Afghan people" for the lunch time blast in the crowded city centre. The bomb was in a vendor's push cart.
Six people were admitted to hospital, five in serious condition, said Abdullah Fahim, a ministry of health official. Several people had superficial wounds, he said.
The blast shattered windows in the justice ministry and damaged several cars.
No one claimed responsibility for the explosion.
Although Afghanistan is experiencing its worse violence since 2001, most of the fighting has occurred in the south and east of the country. Attacks in Kabul are rare.
The surge in violence has been a surprise to the government and its Western backers. About 1,200 people, most of them militants, have been killed in Afghanistan since January. About 60 foreign troops have been killed.
The US had been hoping to reduce its force this year as NATO moved in, but now has about 23,000 troops in the country, the most since 2001.
NATO, which already has troops in the generally peaceful north, west and centre, will soon have 16,000 troops in the country for what was initially billed as a peacekeeping mission.
Meanwhile, police battled insurgents in the Girish district of the southern Helmand province, killing 12 of them, an interior ministry official said.
In another attack, five Afghan labourers working at a US military base were killed in an ambush on their way home from work.
Police said on Tuesday: "Gunmen stopped their vehicle, took them out and opened fire. Five were killed and one wounded."
The labourers worked at the base in the province on the Pakistani border. Kunar is one of the most violent parts of the country where US forces have mounted a series of offensives.