Democratic Republic of Congo opposition presidential candidate Azarias Ruberwa says he will reject the result of Sunday's historic election.
Ruberwa demanded a re-run in some areas
Mr Ruberwa - a former rebel leader who became a vice-president in terms of a 2002 peace deal - said he would use all legal means to annul the results.
He demanded a re-run of elections in some areas. Official election results are not expected for several weeks.
The United Nations hailed the poll as a milestone in DR Congo's peace process.
Mr Ruberwa said he had witness accounts of poll fraud, though he has not yet produced any evidence.
"These irregularities are so significant that they distort the results of the vote across the national territory," a statement from Mr Ruberwa's RCD party said.
As a former rebel leader, Mr Ruberwa retains his own armed forces but before the election he told the BBC that he would not resort to war if he felt the polls had been rigged.
But he warned he would "use democratic means to prevent a false leader from ruling the country".
In the capital, Kinshasa, a TV station close to another ex-rebel leader and Vice-President Jean-Pierre Bemba continues to announce results, despite a warning from the Independent Electoral Commission (CEI) and the Media Authority to stop doing so.
UN Secretary General Kofi Annan welcomed the DR Congo poll - the first democratic election in 40 years - as "a milestone in the country's peace process," according to a statement from his spokesman.
South African President Thabo Mbeki - whose country hosted the peace talks that led to the election - also congratulated the Congolese people on the election.
"I take this opportunity to pay tribute to the Congolese religious community and all political parties for massively mobilising all sectors of society to participate in the making of history," Mr Mbeki said.
In another development, the UN Security Council voted on Monday to extend by a year its arms embargo against militia groups in eastern Democratic Republic of Congo.
The Security Council resolution expressed concern that arms trafficking was fuelling and exacerbating conflicts throughout the region.
"The 15-member body noted that militias have been exploiting the DRC's natural resources to generate income to fund their activities," a UN statement said.
"The problem is most acute in the Ituri, North Kivu and South Kivu provinces in the east of the country, where the arms embargo began in 2003."
The elections were aimed at ending a long civil war, with 32 candidates, including incumbent Joseph Kabila, contesting the presidency.
Congolese describe their election hopes and fears
While many polling stations in the cities have already posted results, the large size of the country makes it unlikely that the finally tally will be available for several weeks.
It is still far from clear whether a second round will be required, or whether a candidate will gain the 50% of the votes needed to win outright, the BBC News website's Joseph Winter reports from Kinshasa.
Over 9,000 candidates stood for parliament. Some 25m voters were protected by 17,000 UN peacekeepers, most of them stationed in the east.
Mr Kabila, who came to power unelected in 2001, has told the BBC he will accept the result of Sunday's presidential elections, even if he loses.
The UN deployed 17,000 peacekeepers during the election
Opposition leader Etienne Tshisekedi and his UDPS party boycotted the vote, and there were reports of violence on election day in Mr Tshisekedi's stronghold, the south-eastern Kasai region.
The presidential candidates include the four vice-presidents who took office in 2003 in terms of a transitional power-sharing deal.
Three of the four vice-presidents are the leaders of former armed factions.
Some opposition candidates accuse Mr Kabila of being backed by the international community, and are already unofficially complaining about what they say are irregularities in the voting, our correspondent says.