Congolese presidential candidate Azarias Ruberwa has alleged "widespread fraud" in Sunday's elections, saying the vote may have to be rerun in some areas.
Speaking in Kinshasa Tuesday, Ruberwa cited cases of ballot stuffing, voter registration problems and intimidation by election officials.
Monitors with the U.S.-based Carter Center reported some problems with the voting process, but said they appeared to be minor.
United Nations Secretary-General Kofi Annan congratulated the Democratic Republic of Congo (DRC) for the largely peaceful manner in which it held its first free elections in 45 years.
Voting was extended through Monday in central Congo after some polling stations were burned Sunday.
Final results from the presidential election are expected in three weeks. Results from the parliamentary vote are expected sooner.
A run-off between the top two presidential candidates will be held October 29 if no one wins more than 50 percent of the vote.
President Joseph Kabila is widely expected to retain power, beating the more than 30 other candidates.
This election was meant to be the DRC's first free and fair vote, following decades of war and unrest in the central African country.
Some information for this report was provided by AFP, AP and Reuters.