The disputed vote threatens to further destabilize poverty-stricken DRC, which is already grappling with a security crisis in the east that has hampered development in the world's top producer of cobalt and other industrial minerals and metals.
Five of President Felix Tshisekedi's challengers in the race called on their supporters to join a march on Wednesday against the poll, which they say was fraudulent and should be annulled.
They vowed to proceed even after the government banned the protest on Tuesday, saying it was intended to undermine the work of the national election commission (CENI) as it compiles results that for now show Tshisekedi with a strong lead.
Police surrounded the headquarters of one of the incumbent's main challengers, Martin Fayulu, where protesters were meant to gather for a scheduled start time of 0900 GMT. Some were in riot gear while others held rifles.
There was no immediate sign of large crowds gathering amid the heavy security presence. But some protesters tried to block roads with burning tires before police intervened with tear gas. People around Fayulu's headquarters also threw rocks at the police, who retaliated in kind.
Tshisekedi "did not win the election, his victory is fraudulent," one protester who gave his name as Jean-Pierre said outside the building.
Government spokesman Patrick Muyaya said the demonstration was banned and that police were taking necessary security measures.
In a sign of tension elsewhere, supporters of a local candidate, who appeared to be losing, briefly blocked roads and disrupted traffic with burning tires in the eastern city of Butembo, before dispersing.
After a sometimes violent campaign, the December 20 vote itself was chaotic, with delayed election kit deliveries, malfunctioning equipment and disorganized voting lists. Violence also disrupted the poll in some places.
The protest organizers have heavily criticized the CENI's decision to extend voting at polling stations that failed to open on election day, calling it unconstitutional and grounds for a full re-run.
Some independent observers have also said the extension undermines the credibility of the poll.
CENI has acknowledged there were delays but has denied that the legitimacy of the election was compromised by extending some voting.
Its latest tally on Tuesday put Tshisekedi well ahead of his 18 challengers, with almost 79% of around 6.1 million votes counted so far.
Around 44 million were registered to vote, although the number of ballots cast remains unknown.
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