Tear gas has been fired in Democratic Republic of Congo at protesters angry that several opposition strongholds have been left out of Sunday's polls.
Electoral officials have postponed voting in three cities until March, citing insecurity and Ebola concerns.
But with the new president due to be sworn in next month, it appears the votes of more than a million people will be discounted.
The opposition accuses the authorities of seeking to rig the ballot.
DR Congo has not had a peaceful and democratic transfer of power since independence from Belgium in 1960.
President Joseph Kabila, who has been in office since 2001, was meant to have stepped down in 2016.
However, the election to choose his successor has been continually postponed, amid unrest and logistical difficulties in a nation with poor infrastructure.
Opposition supporters suspect Mr Kabila is trying to cling on to power. He denies the allegation and is backing former Interior Minister Emmanuel Ramazani Shadary in the election.
Where did protests break out?
Voting was postponed in the opposition strongholds of Beni and Butembo in the east, and the city of Yumbi in the west.
In Goma, the main city in eastern DR Congo and also an opposition stronghold, crowds blocked a road in the Majengo neighbourhood and around the university, the BBC's Gaius Kowene reports from the scene.
Anti-riot police are positioned at some street corners in Majengo, and protesters retreated after tear gas was fired, he adds.
Protests were also reported in Butembo while in Beni, soldiers fired tear gas to disperse protesters, Reuters news agency reports.
The credibility of the election is in doubt following the decision to delay the vote in three cities
So why has voting been delayed in the three areas?
In Yumbi, at least 80 people were killed earlier this month and thousands have fled to neighbouring Congo-Brazzaville amid violence triggered by a dispute over where to bury a traditional chief.
Beni has been affected by an Ebola outbreak that has killed at least 300 people since August. Nearby Butembo has seen attacks on civilians attributed to a Ugandan Islamist militia, the Allied Democratic Forces.
Opposition supporters accused the government of attempting to disenfranchise them, and have vowed to continue with protests to force the electoral commission to reverse its decision.
Moise Katumbi, an exile who is supporting opposition candidate Martin Fayulu, said the postponement was "unjustifiable" and showed that "the regime wants to be in power forever to continue its pillage".
What other issues have there been?
On 21 December, the electoral commission said the vote was being delayed by another week, after admitting that it was not ready. The decision was condemned by opposition politicians.
A delay in delivering voting materials to polling sites after a fire was behind the postponement, officials said.
The blaze destroyed more than two-thirds of the electronic voting machines allocated for the capital Kinshasa, where four million people live, an official said.
About 40 million people have registered to vote.
Meanwhile the government accused Mr Fayulu of instigating election violence. Mr Fayulu's campaign has rejected the charge.
What has been the international reaction?
Representatives of eight African states agreed at a meeting on Wednesday to send a delegation to DR Congo to raise their concerns about the poll with Mr Kabila.
In a statement after a meeting in neighbouring Congo-Brazzaville, the representatives said there was "strong concern over acts of violence" during the presidential campaign, AFP news agency reports.
Angola, Botswana, Congo-Brazzaville, Namibia, Rwanda, South Africa, Uganda and Zambia were represented at the meeting.
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