The victims, including a child and a woman, were killed on Friday night after a hand grenade exploded at a public gathering 15 km north of Yambio town, Justice told Xinhua by phone in Juba on Monday evening.
"On Friday evening at around 11 p.m. local time an identified suspect threw a hand grenade into the night public gathering, killing three people on spot as four others later died of injuries," Justice said.
The police spokesman said 35 people are currently being treated with serious injuries at the Yambio Civil Teaching Hospital.
He said the incident is not linked to armed opposition in the country.
Local authorities arrested the suspect over the weekend and investigations are ongoing, Justin said.