Police say the dead include a Japanese man, a member of President Joseph Kabila's guard and a soldier protecting Kabila's rival Jean-Pierre Bemba.
Tensions ran high in the capital Kinshasa ahead of the announcement that Joseph Kabila and Jean-Pierre Bemba face a second round run-off in the presidential election after neither candidate won an outright majority.
Mr Kabila, the incumbent president, won 44.81 percent of the vote, while Mr Bemba, a former rebel leader gained 20.03 percent, according to the full provisional results.
Election experts said Mr Kabila faced a second round run-off on October 29 against his main rival, Mr Bemba, a vice-president and former rebel leader.
The results showed a turnout of about 70 percent among the DRC's 25 million voters in the 169 electoral districts in this vast central African country.
Skirmishs erupted between guards of the two main rivals and volleys of gunfire rang out elsewhere in the city.
"There were shots fired around the headquarters of the MLC (Bemba's Movement for the Liberation of Congo). Elements of the presidential guard arrived very fast and there was a sustained exchange of gunfire," Mr Bemba's spokesman Moise Musangana said.
"A soldier is dead among the ranks of Bemba's guard," he added.
A western military source and a MONUC official confirmed the death, adding that six people, mainly police officers, had also been injured outside the MLC headquarters and in other clashes around the city.
About 10 more people were wounded in Kinshasa as shooting continued throughout the evening.
At least two army armoured vehicles had taken up position outside Mr Bemba's party headquarters while UN peacekeepers stationed 13 armoured vehicles in front of the CEI's headquarters.
Security had earlier been tightened in the capital after a fraught three-week wait for the electoral commission to announce full provisional results of the country's presidential and parliamentary elections that were held on July 30.
First elections in 46 years
The elections were the country's first free multi-party ballots since former Zaire won independence from Belgium 46 years ago.
Tension remained high in the capital throughout the day and sporadic gunfire ran out for at least three hours.
"The Kabila camp is taking the prospect of a second round very badly. A second round represents the unknown and that makes them scared," an election expert told AFP.
At 35, Mr Kabila is Africa's youngest head of state and few thought he would survive his time in office following recent turmoil and the murder of his father in 2001.
The definitive outcome of the presidential and parliamentary election will be announced by the Supreme Court no later than August 31 after examination of possible objections.
A number of the 33 presidential candidates have already alleged massive fraud.
The elections last month aimed to complete a three-year period of political transition that followed the country's five-year civil war from 1998 to 2003.
The brutal conflict drew in six foreign armies and claimed more than three million lives.