Clashes have erupted between security forces and supporters of a presidential candidate in the Democratic Republic of Congo's capital, Kinshasa.
Many of Mr Bemba's supporters are said to be heavily armed
Police said two civilians were killed in cross-fire after gunfire and explosions were heard near the offices of Vice President Jean Pierre Bemba.
He trails his rival, President Joseph Kabila, with most votes counted in the second round of a presidential poll.
Fighters loyal to the two men clashed after the first round of voting.
At least 23 people were killed in gun battles in Kinshasa after the announcement of first round results in late July.
The first round of elections showed a regional divide, with Mr Kabila gaining a landslide in the Swahili-speaking east, while Mr Bemba got most support in the west, where Lingala is the common language.
Mr Bemba's supporters have repeatedly said electoral fraud has damaged their candidate's chances.
Latest results from the second round run-off give Mr Kabila 61% of the vote, and the former rebel leader Mr Bemba 39%, according to the Independent Electoral Commission (CEI) website.
Saturday's violence is said to have started when Mr Bemba's armed supporters took to the streets near his offices.
Police reportedly fired in the air to disperse the group, who had placed burning tyres in the streets to disrupt traffic.
Gun and heavy artillery fire has been heard, and reports say Mr Bemba's supporters are armed with mortars and rocket launchers.
According to the BBC's Arnaud Zajtman, many of them are high on drugs and half-naked, with a piece of red tissue on their head or their shoulder to differentiate them from the other soldiers.
He says they may be trying to provoke unrest in the capital, knowing that most of the people in the area voted for Mr Bemba in the election.
According to the Reuters news agency, the government has threatened to despatch the army to quell the unrest.
"If this continues, the army will have to intervene to restore order," Interior Minister Denis Kalume told the agency.
UN and EU peacekeeping troops stationed in the city are also on alert but have not intervened.
This year's elections were the first since the end of DR Congo's five-year civil war, in which up to four million people died.
Kabila (above, at left): 61%
Bemba (at right): 39%
Votes counted: 65%
They are also seen as the country's first free elections since independence in 1960.
United Nations observers said the vote was the most significant one in Africa since Nelson Mandela was elected as South Africa's president in 1994.
Turnout in the second round was 67%, with 65% of the votes counted, according to the CEI.
The commission has until 19 November to announce the results and stresses that no "trend projection" can be made on the basis of the provisional results.
Both men have pledged to respect the outcome of the election.
The world's largest peacekeeping force - 17,000-strong - is in DR Congo, tasked with ensuring security.
International observers generally praised the vote as being well-run, despite some disruptions in the north-east of the country.
The election was intended to close the door on decades of dictatorship and conflict.
Counting the votes is a time-consuming process as all the ballot papers had to be transported from sometimes remote locations to compilation centres.
DR Congo is two-thirds the size of western Europe and has just 300 miles of paved roads.
The country's rich reserves of minerals such as gold, diamonds and coltan - used in mobile phones - have attracted a series of armed groups, both Congolese and foreign, intent on looting.