The largest opposition party in the Democratic Republic of the Congo has claimed its leader is the “presumed” winner of last month’s presidential election, as observers identified numerous problems with the vote.

The country’s electoral commission has yet to release the results of the 30 December election, despite an announcement being due last Sunday. But on Tuesday, the Union for Democracy and Social Progress (UDPS) called for transition talks between its candidate, Félix Tshisekedi, and the outgoing president, Joseph Kabila.

The Catholic church – which has significant power in the DRC and had 40,000 observers across the country on election day – announced last week that one candidate had won decisively and it knew who that person was. Polling just before the election predicted a third candidate, Martin Fayulu, would win by a wide margin.

The election was widely acknowledged to have been marred by violence and voter suppression.

A local observer group, Symocel, said it witnessed 52 “major” irregularities in the 101 vote-counting centres it monitored, including people tampering with results. The group said 92% of the centres did not post tally sheets outside, as legally required.

The UDPS secretary general, Marc Kabund, addressed speculation that his party and Kabila’s were considering sharing power.

“As for rumours of a rapprochement between the outgoing president, Joseph Kabila, and the presumed winning candidate of the election, in this case Félix Tshisekedi Tshilombo, the UDPS wants to clarify that must be seen in the context of national reconciliation,” he said.

“Kabila and Tshisekedi have an interest in meeting in order to prepare the peaceful and civilised transfer of power.”

Tshisekedi’s late father, Etienne, was a stalwart of the opposition in the DRC who stood up to Kabila – who has been in power since 2001 – Kabila’s father and predecessor, Laurent, and Mobutu Sese Seko, the dictator who ruled the country for more than 30 years.

Only the Southern Africa Development Community and, in particular, its most powerful member, South Africa, appear to be in a position to exert sufficient pressure on Kabila to accept what appears to be the defeat of his handpicked candidate, Emmanuel Ramazani Shadary.

However, the South African president, Cyril Ramaphosa, has remained silent on the matter, and Pretoria refused to back a UN security council statement that criticised the DRC government for cutting internet access and blocking some of the media.

The South African ambassador to the UN said such a statement could “inflame” the situation.

Ramaphosa’s predecessor, Jacob Zuma, whose supporters in the ruling African National Congress are still numerous, had close ties with the DRC.