Juma Gul Hemat, a provincial police chief, blamed the bus bombing on "enemies of Afghanistan", a phrase used to refer to the opposition Taliban.
"[The bomb] struck a civilian bus and killed innocent people," he said.
Among the civilians killed was Dost Mohammad Arghestani, head of the social affairs department in Kandahar province, who was on his way to work.
He was attacked by two armed men on a motorbike, according to Matiullah Qateh, the police chief.
Qateh said Arghestani's bodyguard was killed in the attack, and his driver was wounded.
Kandahar is one of the main strongholds for Taliban fighters, but drug smugglers, criminals and some tribal rivalries have also contributed to violence there.
Isaf death toll
Three soldiers with Nato's International Security Assistance Force (Isaf) were also killed on Tuesday in Afghanistan when their vehicle struck a bomb, Isaf said.
The location of the blast was not specified.
The 40-nation force, in Afghanistan to help the government fight a growing Taliban-led resistance, did not give the nationalities of its latest casualties, leaving this to their home countries.
"Three Isaf soldiers were killed in eastern Afghanistan [on] October 14 when their vehicle struck an IED, or improvised explosive device," it said in a statement.
About 227 international soldiers have lost their lives in Afghanistan this year, more than the whole of last year.
Most of those killed have died in roadside bomb blasts.
In 2007, a total of 219 foreign soldiers were killed in the country.