Kabila won with 58 percent of the vote compared to Bemba's 42 percent, the country's head of elections said, according to the Associated Press.
"[H]aving garnered the absolute majority of votes in the second round, Mr. Joseph Kabila Kabange is declared president of Democratic Republic of Congo," Apollinaire Malu Malu announced.
Congolese voters have been waiting more than two weeks to see the results of a race that pitted the son of the country's murdered former President Laurent Kabila, against former rebel leader and current Vice President Bemba.
In August, following the results of Congo's first election in more than 40 years, forces loyal to Bemba clashed with Congolese police in street fights that killed at least 30 people.
International peacekeepers, including U.N. forces and troops from the European Union, have since been deployed in the capital Kinshasa to ensure stability. Despite their presence -- U.N. tanks could be seen in the capital Wednesday, the AP reported -- three civilians and one soldier died in fighting on Saturday between the rival political factions.
Kabila called for calm in the city this week and offered Bemba and his supporters a role in the country's development.
"I believe Vice President Bemba and members of his party have a role to play, if not necessarily in the government then in other institutions, because the effort now must be nation-building, reconstruction," he said.
But, although international observers, including some from the Washington, D.C.-based IFES called the election "free and "fair," Bemba's party considers the vote fraudulent and laced with "systematic cheating." Bemba supporters also questioned the credibility of the electoral commission, Reuters reported.
"Our position has not changed," Joseph Olenghankoy, a spokesman for Bemba's Union for the Nation coalition, told Reuters. "These are results that will never be credible."
The electoral commission has not said whether it will conduct an investigation into Bemba's allegations, according to the AP.
Congo's election is the culmination of a peace process that helped end the central African nation's brutal five-year civil war. An estimated 4 million people died in the violence and in a humanitarian crisis that followed.
Kabila, one of Africa's youngest leaders at 35, first took power in 2001 when a bodyguard posing as a member of the presidential security detail assassinated the elder Kabila.