President Félix Tshisekedi has been declared the winner of Democratic Republic of Congo's election, which has been condemned as a "sham" by several opposition candidates demanding a rerun.

The president won about 73% of the vote, with his nearest challenger, Moise Katumbi, on 18%, officials said.

The 20 December election was marred by widespread logistical problems.

It had to be extended to a second day in some parts of the vast country.

About two-thirds of polling stations opened late, while 30% of voting machines did not work on the first day of the vote, according to an observer group.

Millions of people waited for hours before they were able to vote, while some gave up and went home.

The opposition said the problems were part of a deliberate plan to allow the results to be rigged in favour of Mr Tshisekedi, 60.

Several of the main challengers have called for protests after the results are announced.

"We call on our people to take to the streets en masse after the proclamation of the electoral fraud," they said in a joint statement on Sunday.

The army has been deployed in various parts of the capital, Kinshasa, to prevent any unrest, while Mr Tshisekedi's supporters have taken to the streets to celebrate.

The head of the election commission has previously said the opposition candidates wanted a new election because "they know they lost... they are bad losers".

Election chief Denis Kadima acknowledged some irregularities but insisted that the results reflected the will of the Congolese people.

President Tshisekedi will be sworn in for a second term on 20 January.

The son of veteran opposition leader Étienne Tshisekedi, the president was first elected in 2019.

That followed a poll which some observers, including the influential Catholic Church, said was won by former oil executive Martin Fayulu. He came third in this election with 5% of the vote.

None of the other 16 candidates gained more than 1% of the vote.