"The suicide attacker was on a motorcycle, he detonated himself in the middle of a volleyball match," Attaullah Fazli, deputy governor of Paktika province, told AFP.
"A lot of people including some provincial officials and the police chief were there. About 50 people have been killed, and 60 injured, a lot of them seriously."
The attack was in Yahya Khail district of Paktika, a volatile province bordering Pakistan.
President Ashraf Ghani, who came to power in September, swiftly condemned the attack, according to his spokesman. There was no immediate response from the Taliban insurgents, who are responsible for many of the attacks across Afghanistan.
Paktika was also struck by a massive suicide blast in July, when a bomber driving a truck packed with explosives killed at least 41 people at a busy market in Urgun district.
Today's attack occurred on the same day that Afghanistan's lower house of parliament approved agreements to allow about 12,500 NATO-led troops to stay on next year as the national army and police struggle to hold back the Taliban.
US-led NATO combat operations will finish at the end of this year, but the Taliban have launched a series of recent offensives that have severely tested Afghan soldiers and police.
The new NATO mission - named Resolute Support - will focus on supporting the Afghan forces, in parallel with US counter-terrorism operations.