Western embassies in Democratic Republic of Congo urged restraint on Saturday after leading opposition members called this week's election a "sham", some calling for it to be annulled.
Five opposition leaders called in a joint statement for a protest march next Wednesday.
One election officer meanwhile said late voting in a few places in the country's far east had been authorised for Sunday.
In a joint statement Saturday, 12 European embassies in the capital Kinshasa, and the Canadian embassy, called for calm.
"So long as the vote count is continuing, we call on all the parties involved... to show restraint," they wrote.
The US embassy issued a similar call the day before.
Around 44 million people in the nation of 100 million were registered to vote, with more than 100,000 candidates running for various positions.
President Felix Tshisekedi, 60, is running for re-election against 18 opposition candidates.
The sheer scale of the DRC -- roughly the size of continental western Europe -- and its dire infrastructure make elections a stark logistical challenge.
Massive delays and bureaucratic chaos marred the vote on Wednesday, with election authorities struggling to transport voting materials to polling stations on time. Some stations were unable to open at all.
- Protests, fraud allegations -
Officially, the country's electoral commission, Ceni, extended the vote only until Thursday for stations that had been unable to open on polling day.
But ballots were still being cast on Saturday in remote areas, according to some officials, in a sign of continuing difficulties.
And Macaire Kambau Sivikunula, an election official in the North Kivu region in the east of the country, told AFP that Ceni had granted special permission for five voting stations to open on Sunday for voting.
Failure to get voting up and running on schedule had led to him and his family and other election officials receiving death threats, he added.
Five opposition leaders said in a statement Saturday that they were organising a protest march for next Wednesday.
They included gynaecologist Denis Mukwege, the 2018 Nobel Peace Prize laureate; and former oil executive Martin Fayulu.
"We will protest against the irregularities noted during the voting operations," they wrote in a letter to the governor of Kinshasa, where they plan to hold the march.
Five other opposition candidates, including business magnate and former provincial governor Moise Katumbi, released a separate statement calling for the vote to be annulled, saying it had been marred by "massive fraud".
- 'Serious irregularities' -
The poor but mineral-rich central African nation held four concurrent polls on Wednesday to elect the president, lawmakers for national and provincial assemblies, and local councillors.
Ceni on Friday evening released presidential results from the Congolese diaspora, which represents a minuscule proportion of the electorate, indicating a provisional lead for Tshisekedi.
The release of other results expected on Saturday was however postponed until Sunday.
Results from the DRC's 26 provinces are expected to start being released next.
Tshisekedi is considered the front runner in the first-past-the-post presidential election, especially given that the opposition is divided.
The African Union, which conducted an observation mission, said the poll had taken place in an atmosphere of calm but noted major logistical challenges.
Election observers from the US-based Carter Center noted "serious irregularities" at 21 out of 109 polling stations it visited.
On Saturday, a team of Congolese civil-society observers named Regard Citoyen reported that in 21 percent of the polling stations it observed, would-be voters were turned away illegally.
But in 13 percent of polling stations, people either without voting cards or not listed on the voter rolls were allowed to cast ballots, it said.
The Congolese government and Ceni have both rejected accusations that the vote was chaotic.
Ceni also announced an inquiry into "acts of violence, vandalism and sabotage carried out by certain ill-intentioned candidates".