Unzola said shooting began when police raided the home in Matadi, the provincial capital, of Nemuanda Nsemi, his candidate for vice-governor and the spiritual head of Bundu dia Kongo, an anti-government ethnic-based political and religious movement.
General Denis Kalume, the DRC interior minister, has travelled to Bas-Congo to direct efforts to control the violence.
The violence, which erupted late on Wednesday in several towns of the province, appeared to be the worst since Joseph Kabila won last year's presidential election.
Kabila defeated his rival, former rebel leader Jean-Pierre Bemba, in what were the first free elections held in more than 40 years in the vast, former Belgian colony.
The violence erupted after a verbal attack by Bemba on Kabila last week in which he accused the president's camp of buying local assembly members' votes for governors and senators.
A UN official, who asked not to be named, said UN peacekeepers had fired in the air to free UN staff from two vehicles surrounded by about 200 stone-throwing protesters during the violence.
One of the vehicles was set alight.
On Thursday, a hospital administrator at Boma, a town across the Congo river from Matadi, said: "We've got 25 bodies here, including two police officers."
UN Okapi Radio quoted the mayor of Matadi as saying 12 people had been killed in the riverside port, and that more deaths had been reported from Moanda on the Atlantic coast.
But reports of casualties were confused and officials from the UN peacekeeping force in DRC said it was too early to give a definitive death toll.