Congolese opposition leader Martin Fayulu will go to court to challenge results that showed that he lost last month’s presidential election.

Fayulu claims that official results showing that rival opposition candidate Felix Tshisekedi won the Dec. 30 vote were rigged, and he has urged his supporters to protest them. Fayulu topped an opinion poll before the vote, and Western diplomats have said an observer mission run by the country’s influential Catholic Church found that he comfortably garnered the most votes.

“We will introduce an appeal to the Constitutional Court,” Eve Bazaiba, a spokeswoman for Fayulu, said in a phone interview Friday.

Congo is preparing for its first-ever transfer of power via the ballot box, bringing an end to President Joseph Kabila’s 18-year rule. The result, which the Constitutional Court has yet to validate, confounded expectations that Kabila’s protege, Emmanuel Ramazani Shadary, would win. Fayulu was the runner-up, while Shadary came in third, the electoral commission said.

Tshisekedi has praised Kabila for allowing a transition and has given indications that he won’t pursue investigations of Kabila or members of his government.

Any court challenge has little prospect of success, said Claude Kabemba, director of the Johannesburg-based Southern Africa Resource Watch. Kabila appointed three new judges to the court in May, two of whom were well-known allies. “The court is full of Kabila sympathizers,” Kabemba said by phone.

The observer mission run by the country’s Catholic Church, the National Episcopal Conference of the Congo, said Thursday that the electoral commission’s results “do not correspond to the data collected by our observation mission from polling and counting stations.” The conference deployed 40,000 observers for the vote.

Fayulu told supporters in Kinshasa on Friday that the results collected by his coalition’s compilation center show that he won 61 percent of the vote. Electoral commission tallies show he garnered 35 percent, compared with 39 percent for Tshisekedi and 24 percent for Shadary.