The crush occurred in Kikwit on the first night of a festival to commemorate famous Congolese rumba musician King Kester Emeneya, who died last month.
A spokesman for the provincial government said that 21 people had lost their lives and about 12 others were injured. An inquest had been launched into the causes of the tragedy.
Survivors at the general hospital in Kikwit, about 500 km (300 miles) from the capital Kinshasa, said the power cut led to the stampede towards the exits, crushing people in the process.
"When the power went out we tried to contain the crowd that was outside and wanted to enter the stadium, which was already full," a policeman said.
"At the same time those inside wanted to get out, that is why there was a crush which resulted in these deaths".