Gunfire was exchanged in the capital, Kinshasa, between supporters of Joseph Kabila, the president, and Jean-Pierre Bemba, the vice-president and Kabila's opponent in an election runoff.
Witnesses and UN peacekeepers said sporadic gunfire and explosions could be heard for several hours.
Colonel Dominique Mandjenga said: "There were two dead, two civilians killed by stray bullets."
He added that two other people, an 18-year-old student and a policeman, were seriously wounded.
The trouble began when police fired into the air to disperse protesters who burned tires in the streets around the residence of Bemba, an ex-rebel chief who is challenging Kabila in a runoff poll.
The demonstrators were Bemba supporters who are alleging fraud in partial results from an October 29 vote that put Kabila in the lead.
Fighters loyal to Bemba faced off against Kabila troops on the other side of a nearby golf course.
Appeal for calm
Stephane Lescoffit, a UN military spokesman, said it was unclear who fired the first shot. He said he did not know whether fighters were firing in the air or at each other.
Lescoffit said the UN force had issued an urgent appeal for both sides to remain calm.
A contingent of Uruguayan peacekeepers said that fighting had erupted between the two sides, whose forces clashed in August in three days of gunbattles that left at least 23 people dead.
The latest results released by the electoral commission on Friday are being challenged by Bemba supporters.
With nearly two-thirds of the ballots from the October 29 runoff counted, Kabila leads with about 61 per cent of the vote.
Hundreds of Bemba supporters gathered around Bemba's residence.
Jean-Pierre Michel, a 43-year-old unemployed resident of the capital, shouted: "We voted for Bemba, not Kabila. Kabila must go."
Moise Musangana, a spokesman for Bemba, said the vice-president was not at home during the fighting.