"Thirteen people, most of them police, have died in the bomb blast in Kandahar city and were taken to the Kandahar public health hospital," a doctor on duty, who gave his name as Mamoon, said.
"Eleven are wounded, most of them in critical condition," he added.
The interior ministry confirmed the blast was caused by a bomb.
A destroyed motorbike was at the scene of the blast, which was splattered in blood, according to an AFP journalist.
"We think the bomb was attached to a motorbike that was parked outside the building," police commander Bashir Khan said as he surveyed the site of the blast.
"There are many casualties."
Police and Canadian soldiers with the US-led coalition force in Afghanistan immediately sealed off the area.
Kandahar, the birthplace of the Taliban movement that rose to control most of Afghanistan by 1996, has seen the worst of a rash of suicide and car bomb attacks that have claimed dozens of lives in past weeks.
Around 1,700 people were killed in insurgent-linked violence last year, many of them militants killed by Afghan security forces or by troops from the US-led coalition helping the government to hunt down insurgents.