Opposition presidential candidate Martin Fayulu has warned election officials in the Democratic Republic of Congo not to “disguise the truth” as tensions mount over the delayed result.
Mr Fayulu said the “Congolese people already know” the result of the vote, which took place on 30 December.
A local observer group said it had witnessed “major irregularities” at counting stations.
President Joseph Kabila is due to step down after 18 years in office.
The election outcome was initially expected to be announced on Sunday.
Anti-riot police have been deployed in the capital, Kinshasa, and access to the offices of the election commission has been blocked, reports the BBC’s Louise Dewast from the city.
Amid growing concerns that violence could break out, Zambia’s President Edgar Lungu and his South African counterpart Cyril Ramaphosa met on Wednesday to discuss the situation in DR Congo.
Details of the meeting, held in South Africa’s capital Pretoria, have yet to be released.
South Africa is a key ally of Mr Kabila, while Zambia hosts more than 60,000 refugees who have fled conflict in DR Congo.
Mr Kabila has promised that the polls, which were supposed to have taken place two years ago, will be the mineral-rich central African state’s first orderly transfer of power since it gained independence from Belgium in 1960.
His preferred successor, former interior minister Emmanuel Shadary, is facing strong opposition from Mr Fayulu, an ex-oil tycoon, and Felix Tshisekedi, son of a veteran opposition leader.
On Tuesday, Mr Fayulu said DR Congo’s National Electoral Commission (Ceni) must “publish the provisional results of the presidential election in the near future,” adding: “The election results are not negotiable.”