A team of international election observers says the results of last month's disputed presidential election in the Democratic Republic of Congo were “mismanaged” and lack credibility.

The U.S.-based Carter Center said the vote, in which President Joseph Kabila was re-elected, was marred by wide variations in the quality of vote counting.

In a statement issued late Saturday, the center said there was evidence that results from nearly 2,000 polling stations in the capital, Kinshasa, were lost. It said multiple locations around the country reported 99-to-100 percent voter turnout, with most or all of the votes going to the incumbent president.

Congo's electoral commission declared Friday that Mr. Kabila was the winner of the November 28 poll. However, his top challenger, longtime opposition leader Etienne Tshisekedi, has rejected the results and declared himself president.

A spokesman for Tshisekedi on Sunday said he is hopeful the international community will mediate a solution to the crisis, adding that any protest will be non-violent.

French Foreign Minister Alain Juppe warned that the situation is becoming explosive, and he urged both sides to avoid violence.

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 city.