The Democratic Republic of Congo's Supreme Court has begun hearing a lawsuit seeking to annul the presidential election that returned incumbent Joseph Kabila to power.
Opposition candidate Vital Kamerhe, who finished third in the vote, filed the lawsuit, in which he claims the poll was rigged in favor of Kabila.
Kamerhe, who was present for Thursday's hearing, said some ballots were pre-marked for the president, and said the electoral commission reported false results.
The official tally from last month's poll showed Kabila winning 49 percent of vote, well ahead of second-place finisher Etienne Tshisekedi, who had 32 percent.
Tshisekedi has rejected the results and proclaimed himself president.
International election observers reported numerous irregularities during both the vote and the counting process. On Wednesday, the U.S. State Department said the election was "seriously flawed." However, spokeswoman Victoria Nuland said it was not clear whether the problems were enough to change the outcome of the election.
Kabila has said there is no doubting the credibility of the poll and urged Tshisekedi to use the courts to pursue any challenge to the results.
Violent protests and looting left four people dead in Kinshasa following the announcement of the election results, but a heavy mobilization of police and security personnel has since restored a tense calm to the capital.
The presidential and legislative polls were only the second free elections in Congo since the nation was torn apart by several years of warfare that ended in 2003.