The toll was a provisional estimate, Pierrot Rombaut Mwanamputu said in an emergency broadcast that interrupted programming on public television.
He also said two police officers had been "lynched" and were in critical condition.
Sustained gunfire had been reported earlier close to the central prison in the Congolese capital, where more than 4,000 people escaped in May, and in the districts of Matete and Ndjili, residents and activists said.
"There has been gunfire around Makala prison for about the last hour. There's no traffic and the streets are empty," Emmanuel Cole, a campaigner for prisoners' rights, said from the nearby district of Selembao.