Other 19 people were wounded in the blast in a car park outside the police building, the local government said.
The blast – thought to have been caused by explosives hidden inside a parked car and detonated remotely – was strong enough to shatter nearby windows.
No-one has yet claimed responsibility for the attack.
Kandahar is the largest city in southern Afghanistan and the province is considered to be the spiritual homeland of the Taliban.
Militants frequently launch attacks against government and military targets in the province, despite intensive efforts by NATO-led forces in recent years to improve security there.
The United Nations said on Saturday that civilian deaths in Afghanistan had risen for the fifth year in a row – with 3,021 deaths in 2011 compared with 2,790 in 2010 and 2,412 in 2009.
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