DR Congo's President Felix Tshisekedi has won a second term in office with a landslide victory, according to provisional results announced Sunday, in a vote opposition leaders have dismissed as a "sham".

Provisional results from the single-round presidential ballot, declared by the country's electoral commission Ceni, showed that Tshisekedi had won 73 percent of the vote. Turnout was 43 percent.

The president, flanked by his wife Denise and his mother, appeared on a balcony at his campaign headquarters in the capital Kinshasa to address supporters who braved the rain to hear him speak.

"I have been re-elected president of all Congolese," Tshisekedi, dressed in a white shirt and cap, told cheering supporters. "It's in this spirit of openness that I will exercise this second mandate." 

Moise Katumbi — a wealthy businessman, football club owner, and former provincial governor — was the runner-up with about 18 percent. 

The Democratic Republic of Congo's Constitutional Court is expected to confirm the provisional results on January 10. 

Tshisekedi, 60, first came to power in January 2019 after a disputed election that many observers said he had in fact lost. 

Martin Fayulu — who says he was robbed of the last presidential election in 2018 — also contested this year's poll but in the end won about five percent of the votes. 

The 20 remaining candidates, including Denis Mukwege, who won a Nobel Peace Prize for his work with female victims of wartime sexual violence, were hovering around one percent or less.

Nine opposition candidates — Mukwege, Fayulu, and Katumbi — signed a declaration on Sunday rejecting what they termed a "sham" election and called for a re-run. 

Fayulu told reporters in Kinshasa the results "are a masquerade. This must not be accepted". 

Tresor Kibangula, a political analyst at the Ebuteli research institute who spoke to AFP before the full provisional results were released, said Tshisekedi's vote tally "is way beyond all expectations".  

"His dynamic campaign worked" but his scores in some regions "raise questions about the impact of the irregularities that were observed".

More than 40 million people out of the 100 million inhabitants of the huge country were registered to vote on December 20 for president, as well as for national and regional lawmakers and municipal councilors.

Voting was officially extended by a day to account for problems, and continued for days afterward in remote areas, according to observers.

One Catholic-Protestant observation mission said it "documented numerous cases of irregularities susceptible to have affected the integrity of the vote".

The United States called for peaceful and transparent resolution of any election disputes after Tshisekedi was declared the winner.

"Any election disputes should be resolved peacefully and in accordance with Congolese electoral law," a State Department spokesperson said on Sunday.

About 15 embassies have called for "restraint" in the poor but mineral-rich country where post-election tensions have been common. 

Authorities say they have taken steps to prevent unrest, especially in the mining areas of the southeast that are Katumbi's stronghold.

They also stress that any electoral disputes must be presented to the Constitutional Court.

But opposition leaders say they have no confidence in the court or Ceni, which they argue is subservient to the government. 

Security was tighter in Kinshasa on Sunday, according to AFP reporters, as well as in the southeastern city of Lubumbashi.