"An IED (improvised explosive device) struck a bus, 20 civilians were killed," said Haji Mosa Rasooli, accusing the Taliban of being behind the blast.
The Islamist militants, who have been waging a near decade long insurgency against the Afghan government and foreign forces, were not immediately contactable for comment.
The blast occurred in the remote and volatile southwestern province of Nimroz. It took place at around 4:00pm (1130 GMT) in the region's Khash Rod district on the main highway linking Nimroz to Kandahar, the de facto capital of southern Afghanistan.
The incident came on the same day as a father, a mother and their four children were killed in southern Afghanistan when a roadside bomb ripped through their car.
The family were travelling to Lashkar Gah, the main town in Helmand province, when they were killed, provincial spokesman Daud Ahmadi said.
"Four children, one female, and one male were killed in the sedan car en route to (the provincial) capital," he added.
Although the attack bore the hallmarks of the Taliban, who frequently plant roadside bombs in Afghanistan''s restive regions, the militant Islamist network declined to comment on the attack.
Civilians are the biggest casualties in the near 10-year war in Afghanistan, where 150,000 foreign forces are stationed.