Former warlord Jean-Pierre Bemba was expected to appeal to the Democratic Republic of Congo's top court on Monday after he was banned from running in upcoming presidential elections.
Bemba and five other candidates, including three former prime ministers, were barred by the National Independent Electoral Commission (CENI) late on Friday.
The move sent shock waves through the country's volatile politics ahead of the twice-delayed elections, triggering angry accusations of a fix.
A major rival to President Joseph Kabila, Bemba returned to the DRC on August 1 to file his candidacy after he was acquitted by the International Criminal Court on war crimes charges.
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But he remains convicted by the International Criminal Court for bribing witnesses - a crime for which he was handed a year-long prison term and a 300 000 euro fine.
It was this conviction that CENI invoked as a reason to bar him from the December 23 vote.
Bemba - who once headed a powerful militia since transformed into a political party, the Movement for the Liberation of the Congo (MLC) - indicated he would take the legal route to contesting the election.
"Let's see what happens at the Constitutional Court," he told France 24 television, adding, however, "as long as the court is independent".
But a pro-democracy campaign group, Lucha, mocked the chances of success, branding the Constitutional Court as stacked with Kabila loyalists. "The Constitutional Court's verdict is known in advance," it said on Twitter.
Three of Kabila's former prime ministers - Samy Badibanga, Adolphe Muzito and Antoine Gizenga - were also banned from running, as was the sole woman candidate, Marie-Josee Ifoku Mputa.
Gizenga, a 92-year-old figure from the time of the DRC's independence from Belgium in 1960, was ruled out on the grounds that he had not signed the candidacy forms.
Muzito was excluded because he was a dissident member of a party, Palu, which contested that he was its candidate.
Badibanga and Ifoko Mputa were banned because they had acquired foreign nationality - dual citizenship is illegal under DRC law.
In a statement signed by Bemba and exiled politicians Moise Katumbi and Felix Tshisekedi, opposition groups urged Kabila to free up the electoral process and "stop giving directives to the election commission".
Government spokesperson Lambert Mende dismissed any idea of interference, saying that "legal norms" determined CENI's decisions.
A country of some 80 million people, DRC has never known a peaceful transition of power since gaining independence in 1960.
A long tradition of coups and wars has stirred fears of a plunge back into war if the December vote goes awry.
Kabila, 47, has held office since 2001, taking over from his father, Laurent-Desire Kabila, who was assassinated by a bodyguard.
He has stayed in office since reaching the end of his two-term limit in December 2016 thanks to a caretaker clause in the constitution.
He kept the country in suspense for months over his political plans, ending the uncertainty on August 8 by saying he would not run again.
Kabila has thrown his support behind close ally Emmanuel Ramazani Shadary, a former interior minister and loyalist - a move that has stoked accusations that he wants to remain the power behind the throne.