The United Nations' biggest peacekeeping operation, some 17,500 strong, is currently deployed in the vast former Belgian colony.
The first free elections held in more than 40 years were aimed at ending years of war and chaos.
Provisional results from 159 out of 169 constituencies, published late on Monday, gave Kabila an 18-point lead over Bemba, the former rebel leader who served as Kabila's vice-president in a transition government.
|"The Union for the Nation will not accept an electoral holdup that aims to steal the victory from the Congolese people."|
Bemba supporters' statement
Bemba's Union for the Nation coalition has denounced "systematic cheating" in count, accused the electoral authorities of publishing results which "wrongly" indicated a Kabila victory.
"The way in which the [electoral commission] has distilled the publication of partial results... wrongly aims to credit that Joseph Kabila is on the way to winning the second round of presidential elections," said Vincent de Paul Lunda-Bululum, a Union spokesman.
"This behaviour casts serious doubts on the seriousness and credibility of the Independent Electoral Commission," the Bemba camp said in a statement.
It said its own calculations showed Bemba ahead with 52.5 per cent of the vote.
"The Union for the Nation will not accept an electoral holdup that aims to steal the victory from the Congolese people," Bemba's supporters said.
The defiance of Bemba's coalition comes as UN-backed efforts were underway to re-deploy around 1,000 soldiers loyal to the vice-president to a camp outside the capital, to minimise the risk of further clashes.
A UN official said Bemba proposed the idea, but several of his soldiers told reporters: "They want us to leave, but we will refuse. If we have to fight again, we will".
Bemba's coalition said that if their allegations of cheating were proven then it would not feel obliged to respect accords made with the Kabila side to peacefully accept the election results.
Preliminary results put President Kabila ahead
in the run-off polls
This stand raises serious issues for UN and EU peace-keepers who are trying to prevent DR Congo slipping back into violence.
Fighting between soldiers from the two camps killed at least 30 people in August, after first round of voting in which Kabila gained the most votes, but not enough to carry the election outright.
The elections were the culmination of a peace process ending Congo's 1998-2003 war.
The conflict had produced a humanitarian crisis that has killed some four million people and aid workers say hundreds of Congolese still die every day from violence, hunger and disease.
The Independent Electoral Commission's president, Apollinaire Malu Malu, rejected Etsou's statement as "dangerous".