Kabila won with about 58% of the vote, compared with nearly 42% for Bemba, said Apollinaire Malu Malu, chairman of the Independent Electoral Commission.
Bemba has disputed results from the Oct. 29 vote for days and his forces fought police and troops loyal to Kabila on Saturday for several hours in the capital, leaving three civilians and one soldier dead. Earlier Wednesday, Bemba's fighters refused a request by U.N. peacekeepers to return to military barracks in the city.
The 35-year-old Kabila looked set for victory since earlier this week, when results showed him with an insurmountable lead. No official winner had been declared by the electoral commission until Wednesday night, when Malu Malu made the announcement on state television.
The U.N. mission deployed tanks onto Kinshasa's streets Wednesday, part of stepped-up security across Kinshasa that has also included a greater presence of European peacekeepers and police.
The electoral commission had said it would look into Bemba's complaints alleging fraud, but Malu Malu made no reference to the investigations.
Congo may one day look back at the election as the difficult birth of an era of peace, democracy and prosperity after decades of colonial rule, dictatorship and war that devastated what should be among the richest countries in Africa, if not the world. But for now, it is far from clear that supporters of the political rivals won't resort to violence.
"The big hope is that people want the elections, they want peace, they want to get on with their lives in circumstances that promise a better future," said William Lacy Swing, the U.N. secretary-general's special representative to the Congo.
"The vast majority of people are tired of war, tired of violence, they want peace and they are not easily going to be involved in acts that will disrupt order," he said. "I honestly believe that after these elections, one can truly say that this country is another country."
International observers have said voting was largely free and fair, and the election commission has rejected accusations its count has been skewed by fraud.
When results of the first presidential round were announced in August, the two sides battled it out on the streets of Kinshasa for three days, killing 23 people.
White U.N. personnel carriers with mounted submachine guns and truckloads of troops remained overnight in front of Bemba's house on the capital's main June 30 Boulevard. U.N. military spokesman Lt. Col. Stephane Lescoffit said about 100 troops had been stationed there.
On Tuesday, Bemba's spokesman said, a Congolese Armed Forces officer led a convoy of 16 trucks to the house and demanded that 300 members of Bemba's bodyguard accompany them to be confined to barracks on the outskirts of town. They refused and the trucks left, said the spokesman, Moise Musangana.
Army spokesman Col. Leo-Richard Kasonga said he couldn't immediately comment on the demand that Bemba's forces be confined to barracks, which appeared to have been made without consulting the U.N. peacekeepers.
Musangana said Bemba had about 1,500 armed fighters at his home, while the presidential guard numbered up to 15,000.
On Tuesday, Bemba's supporters challenged the vote count and threatened to tear up agreements to use dialogue and not violence to resolve disputes.
International observers have reported some problems in the largely peaceful elections, but largely agree that the exercise was free and fair ? a tremendous achievement given the size and population of the country, its primitive infrastructure, lack of institutions, and the fragility of a peace that still has not taken hold in the east.
The Central African nation is the size of the United States east of the Mississippi, but has only 300 miles of paved roads, posing huge logistical problems for the U.N. mission that spent nearly $500 million on the vote, the biggest election the world body had ever helped organize.
"There have been issues but, technically speaking, I really believe it was free and fair," said Carmina Sanchis-Ruescas of the Washington-based IFES, which aids elections around the world. "There always some errors and mistakes, but not to the extent that we could claim that it has not been free and fair."
The names of some voters didn't make it onto the lists of eligible voters because of technical glitches, while up to 1.5 million poll workers and observers were allowed to vote outside their home districts, raising concerns.
The country's top Roman Catholic prelate, Cardinal Frederic Etsou, accused the Electoral Commission of publishing false results.
But the biggest national observer mission, set up by the Catholic Church, found no evidence the count was rigged.
"We saw some irregularities, but we cannot speak of fraud," said Henri Nuyiya of the Coordination for a Successful Transition, which mustered 50,000 election volunteers. "If there were a few people who voted in places where they were not registered, this would not have affected the results of the elections."
Both candidates had representatives at polling stations who were given copies of the results on the day of balloting, said Colin Stewart of the Atlanta-based Carter Center, which also sent observers.
That measure, he said, "constitutes a very strong measure of transparency and virtually eliminates any possibility of significant fraud in the compilation process."