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 favor of Mr. Tshisekedi, 60.

Several of the main challengers have called for protests after Sunday's announcement.

"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.