Riot police deployed at the electoral commission headquarters in Kinshasa on Wednesday amid fears of a disputed result in Democratic Republic of Congo's presidential election marked by accusations of vote fraud.

The tally could be announced later in the day after the commission (CENI) met all night and into the morning.

Police also took up positions along the city's main boulevard, as Congolese feared about possible violence amid suspicions that President Joseph Kabila's government was negotiating a power-sharing deal with one opposition candidate.

The December 30 poll was meant to lead to the vast central African country's first democratic transfer of power in its 59 years of independence, but a disputed result could trigger the kind of violence that erupted after the 2006 and 2011 elections and destabilise Congo's volatile eastern borderlands.

In Pretoria, South African President Cyril Ramaphosate and his Zambian counterpart Edgar Lungu met on Wednesday and urged CENI to speedily release the results to maintain stability.

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Last week, South Africa, long a Kabila ally, joined with Russia and China at the UN Security Council to block the release of a statement proposed by France that would have welcomed the holding of the vote but criticised the government's decision to cut access to the internet and some media outlets.

The election commission announced on Tuesday evening it had initiated "a series of evaluation meetings and deliberations, at the end of which it will proceed to the publication of provisional results from the presidential election".

A CENI source and a diplomat said they expected results to be announced later on Wednesday. Another diplomat, however, said that not all the vote tallying had been completed and that the announcement might have to wait until Thursday.

Kabila is due to leave office this month after 18 years in power - and two years after the official end of his mandate. He backed his former interior minister, Emmanuel Ramazani Shadary, in the election.

Shadary was competing against two main opposition candidates, businessman Martin Fayulu and Felix Tshisekedi, the president of Congo's largest opposition party.

Tshisekedi's camp, which says it expects to win, said on Tuesday that it had met with Kabila's representatives to ensure a peaceful transfer of power, although Kabila's camp denied any such meetings had occurred.

Supporters of Fayulu, who had a healthy pre-election poll lead, have voiced suspicions that Kabila may be looking to negotiate a power-sharing agreement with Tshisekedi if Shadary loses.

On Tuesday, Fayulu and six other presidential candidates issued a statement saying that the results "cannot be negotiated".

Domestic election observers say they witnessed serious irregularities on election day and during vote tallying, although a regional observer mission said the election went "relatively well".