Four opposition candidates in the Democratic Republic of Congo's election say it should be cancelled because of fraud and violence.
They include Vital Kamerhe, who said the rigging was on a large scale and "deliberately planned" with pre-marked ballot papers.
He was a close ally of President Joseph Kabila before breaking away in 2009.
Many voters were unable to cast their ballots on Monday and so polling was extended until Tuesday in some areas.
However, the BBC's Christophe Pons visited one such polling station in the capital, Kinshasa, and found that voting had still not taken place by Tuesday lunchtime.
Votes have been counted in some polling stations, but it is not clear when results would be announced.
Election officials are only now starting to organise the transport of these results to regional tallying centres, where provisional results can be announced, correspondents say.
'Fictitious polling stations'
Angry residents told him that there were 1,300 registered voters but only 100 ballot papers had been received.
Mr Kamerhe sent a letter to the election commission and international bodies, saying the vote should be annulled.
"There can be no doubt as to the scale of the fraud, deliberately planned by those in power with the connivance of the national election commission," he said.
"Police chased witnesses from polling stations before counting could start."
Mr Kamerhe's aides also accused the UN peacekeeping mission in DR Congo of showing bias towards Mr Kabila.
"These elections must quite simply be annulled," the letter said.
The Open Society Initiative of Southern Africa, a non-governmental organisation which deployed 5,000 observers to polling stations, also expressed concerns about irregularities, the AFP news agency reports.
"The irregularities are so widespread it will be difficult for anyone to ignore and say they had no impact on the integrity of the vote," its country director, Pascal Kambale, is quoted as saying.
He said millions of of voters had been turned away from polling stations after being told they were at the wrong stations.
"A more worrying sign of a probable rigging attempt were a number of already-filled-in ballot papers that were discovered by people across the country," Mr Kambale is quoted as saying.
Three other candidates, including Senate speaker Leon Kengo, have also called for the results to be declared null and void.
In a joint statement, they said they had uncovered "fictitious" polling stations and pre-marked ballot papers.
They said they attached "no credibility" to the vote and "demand the invalidation, pure and simple, of these elections given the breaches and irregularities".
Our reporter says Mr Kamerhe's decision is the most significant as - along with Union for Democracy and Social Progress (UDPS) leader Etienne Tshisekedi - he was seen as one of the main opposition candidates.
The UDPS has made similar accusations but it has not backed the call for the results to be nullified.
It said it was confident that Mr Tshisekedi would win, despite the irregularities.
Mr Kabila is running for a second term against 10 other candidates.
More than 18,000 candidates are contesting 500 parliamentary seats.
Our correspondent says many people who were unable to cast their ballots are angry.
Some European Union election observers were withdrawn from polling stations on Monday for their own security, Reuters says.
At least four people died after armed men - suspected to belong to a secessionist movement - attacked two polling stations in the southern mining city of Lubumbashi, a stronghold of Mr Kabila.
In the opposition stronghold of West Kasai, 15 polling stations were reportedly set on fire by voters angry at long delays. In the same province, there were unconfirmed reports of ballot boxes being full as polling opened.
After decades of conflict and mismanagement, DR Congo, a country two-thirds the size of Western Europe, has hardly any functioning transport infrastructure such as roads or railways.
The UN peacekeeping mission is using its helicopters to deliver voting materials to areas which have not yet voted, such as the central Bandundu province, Reuters reports.
There had been calls for the vote to be postponed but on the eve of polling, the head of the election commission said everything was 99% in place.
There were reports of lengthy delays at polling stations on Monday, with some voters telling the BBC they were unable to cast their ballots either because they could not find their names on the electoral register or because someone had already voted in their place.
Late on Monday, election commission spokesman Matthieu Mpita said voting would take place on Tuesday in at least 400 polling stations across the country but that the polls could also be extended in other areas.
"Voters at polling stations that never received ballots and which have not yet opened should await the delivery of the materials," he is quoted by the AP news agency as saying.
"Voters who are at sites where ballots ran out and where the vote had to be interrupted for whatever reason are asked to stay calm and await further instructions."
Meanwhile, the UN security council has added one of the parliamentary candidates, Ntabo Ntaberi Cheka, to its sanctions list, AFP reports.
Mr Cheka, the head of a militia group, is wanted for allegedly organising mass rapes in eastern DR Congo in 2009.
The French, UK and US missions to the UN said Mr Cheka would be subject to a worldwide travel ban and assets freeze, AFP reports
"Our missions strongly encourage the Congolese government to implement the existing arrest warrant currently outstanding against Cheka," they said in a joint statement, AFP reports.