DR Congo's Constitutional Court said it would start hearing Tuesday an appeal against presidential election results that gave victory to opposition leader Felix Tshisekedi.
"The Constitutional Court will start examining the appeal by Martin Fayulu tomorrow, from 9.30am [0830 GMT]," press officer Baudouin Mwehu told AFP on Monday.
Fayulu, likewise from the opposition, was declared runner-up in the December 30 poll to choose a successor to President Joseph Kabila, in power for the last 18 years.
He says the results released last Thursday, which are provisional, were an "electoral coup" and is demanding a recount.
Tshisekedi was credited with 38.57 per cent of the vote, against 34.8 per cent for Fayulu.
Emmanuel Ramazani Shadary, the candidate backed by Kabila, came a distant third with 23.8 per cent.
In the few opinion polls conducted before the vote, Fayulu had been tipped as clear favourite. He claims he garnered 61 per cent of the ballot.
Fayulu filed his appeal on Friday. The court has a week to study the request before giving its ruling, with the new president scheduled to be sworn in on January 22, according to Corneille Nangaa, head of the Independent National Election Commission (CENI).
On Sunday, Southern African Development Community (SADC), a bloc that includes Angola and South Africa, called for a unity government and urged a recount to "provide the necessary reassurance to both winners and losers."
The country's long-running political crisis erupted two years ago when Kabila refused to step down at the end of his constitutional term in office, sparking protests which were brutally repressed.
The vast, unstable country has never had a peaceful transition of power since gaining independence from Belgium in 1960.
It became a battlefield for two regional wars in 1996-7 and 1998-2003, and the last two presidential elections, in 2006 and 2011, were marked by bloody clashes.
The influential Roman Catholic Church, which says it deployed 40,000 observers to monitor the poll, has also dismissed the official outcome as not reflecting the true result.
But it has held back from saying who, in its opinion, was the victor.