The incident happened late last night -- the same day that a suicide bomber killed 10 policemen in the northern city of Kunduz, taking the toll to 18 in a single day.
The detainees had been arrested on suspicion of planting a homemade bomb, officials said. "Our police had an operation in Pero Qalacha area last night. They detained several suspected insurgents," Jawed Faisal, the Kandahar provincial governor's spokesman, told AFP.
"On the way back to the city (centre), their vehicle struck an IED (improvised explosive device). Eight police were killed and three suspects that they had detained during the operation were killed," Faisal said.
Six other officers and one suspect on board a second vehicle were wounded in the blast, he said.
Police spokesman Ghorzang Afridi described the bomb as "very, very powerful", saying it was an artillery shell attached to a detonator and possibly set off remotely.
The officials blamed the bombing on the "armed opposition", a reference to the Taliban who have waged an insurgency against the Western-backed government since being ousted from power in a US-led invasion in late 2001.
The Taliban were also accused of yesterday's suicide bombing in Kunduz which killed 10 officers, including the head of a counter-terrorism unit and the provincial head of the traffic police.
Afghan security forces are increasingly targeted by the Taliban as they take greater responsibility for the war ahead of the withdrawal of US-led NATO combat troops next year.