A spokeswoman for the group denounced what she called a policy of cheating. She said the votes for Mr. Bemba have been systematically decreased.
Electoral officials say they will investigate the complaint.
Meanwhile, the two presidential candidates renewed a call for their supporters to remain calm as the country awaits the results of the October 29 election.
Mr. Bemba and incumbent President Joseph Kabila said in a joint statement late Tuesday after meeting in Kinshasa that they would not challenge the election results by force.
Violence broke out following the first round of voting in July, when the private armies of the two candidates clashed for three straight days in Kinshasa. The U.N. mission in Congo said the battles killed some 40 people.
The electoral commission said this week that partial results from October's run-off election show President Kabila in the lead. However, they stress the results are provisional and are subject to change. Final results are expected November 19.
Some information for this report was provided by AP and Reuters.