Congo's veteran opposition chief Etienne Tshisekedi on Sunday called for elections to be held this year and for President Joseph Kabila to step down as scheduled on December 20.
Tensions have been growing in the mineral-rich but troubled state over fears that Kabila, in power since 2001, may try to extend his rule with a third term, beyond the constitutional maximum of two.
Speaking at a rally in Kinshasa, Tshisekedi warned him not to try, saying it would be "high treason" if the electoral process were not launched on schedule in September.
After a two-year absence due to ill-health, Tshisekedi, 83, returned Wednesday to the Democratic Republic of Congo to a warm welcome from supporters.
He told supporters that September 19 was the "first red line which must not be crossed".
"The electoral body must be convened (by that date) for the presidential election. If it is not, high treason will be proved in the person of Mr Kabila, who will take responsibility for the misery of the Congolese people," Tshisekedi said.
"From that moment, his three-month notice period on the presidential palace begins. On December 19 the notice expires and on the 20th the house must be free," he added, to rousing cheers.
An immensely popular figure who emerged as a leading dissenting voice as far back as the 1980s, when he was a critic of strongman Mobutu Sese Seko, Tshisekedi recently accomplished the rare feat of uniting the Congolese opposition.
Congo's opposition has never before managed to forge a common front against Kabila, who beat Tshisekedi in the last presidential election in 2011.
Last month, another leading light of the opposition, Moise Katumbi, was sentenced in absentia to three years in jail for property fraud.
The presiding judge in the case has since claimed she was pressured by the authorities into signing off on a guilty verdict, to ensure Katumbi would be ineligible to run for office, according to a letter seen by AFP.
Tshisekedi travelled to the rally in an open jeep escorted by a swarm of motorbikes along a route thronged with supporters and draped with the flags of various opposition parties.
One group of youths carried a coffin daubed with anti-Kabila slogans.
At the rally itself, opposition supporters waved banners reading "Change is now," and "No dialogue without the release of political prisoners."
"We voted for Tshisekedi in 2011 but still the international community imposed Kabila on us," said one supporter who gave his name only as Martin.