An agreement among opposition parties in the Democratic Republic of the Congo to field a single candidate in a forthcoming presidential election has collapsed, significantly weakening efforts to defeat the ruling party candidate backed by the long-time president, Joseph Kabila.

Representatives of the country’s opposition parties announced in Geneva on Sunday that they had collectively chosen the widely respected parliamentarian Martin Fayulu to face the ruling party candidate, Emmanuel Ramazani Shadary, in December.

But within 24 hours, the two leading opposition parties said they were withdrawing from the pact.

The news of the deal prompted protests by supporters of Felix Tshisekedi, the leader of the biggest opposition party, the Union for Democracy and Social Progress.

Tshisekedi, whose father led the political opposition to Kabila until his death in February 2017, said: “I have realised that the action we took in Geneva was not understood by the party base.”

Supporters of Martin Fayulu celebrate in Kinshasa after the opposition coalition chose him as its candidate. Photograph: Kenny Katombe/Reuters

Another prominent opposition party, the Union for the Congolose Nation, led by Vital Kamerhe, also said it was withdrawing from the pact.

The news will concern many onlookers who had hoped the deal meant the election would lead to a real contest between Shadary and the opposition.

Though government supporters have said “victory is certain”, diplomats in Kinshasa said they believed there was still a possibility opposition parties may rally behind Tshisekedi or another candidate.

Kabila, who took power in 2001, is banned by the constitution from running for a third term. His candidacy was resisted by the US and the EU, as well as significant regional players.

Shadary is a Kabila loyalist without a following of his own. He is under EU sanctions for his alleged involvement in human rights abuses.

The election remains highly controversial. There is deep concern about the introduction of a new electronic voting machine and two of the biggest opposition names are unable to stand.

Supporters of Felix Tshisekedi, the leader of the biggest opposition party, protest in Kinshasa. Photograph: Kenny Katombe/Reuters

Jean-Pierre Bemba, a former warlord and vice-president who returned to the DRC last week after being acquitted of war crimes by the international criminal court, has been ruled out on legal grounds, while Moïse Katumbi, a wealthy businessman and former governor of the province of Katanga, has been prevented from returning from self-imposed exile in Europe. The multimillionaire faces court cases in DRC on charges that he says are trumped up.

Both were present in Geneva for the talks.

An opinion poll in July, before Kabila came out to back Shadary, showed opposition leaders were favoured by about 70% of voters, but the ruling party has a significant financial and institutional advantage.

Kabila took over from his father, Laurent-Désiré Kabila, who was assassinated by a bodyguard. He has remained in power despite his second term ending in 2016, under a constitutional clause that enables a president to stay in office until a successor is elected.

Many provinces are in the grip of armed conflict and millions have had to flee from their homes. Officials in the DRC claim only 230,000 people have been displaced, which is a fraction of the UN’s estimate of 4.5 million. There is deep humanitarian need throughout much of the country.

The DRC, which has significant natural resources, has never known a peaceful transition of power since it gained independence from Belgium in 1960, and some experts fear the election will trigger a bloody conflict.