A presidential election in the Democratic Republic of the Congo due to take place on Sunday has been delayed until 30 December, the country’s electoral commission has announced, citing problems caused by a recent fire that destroyed 80% of the voting machines in the capital, Kinshasa.

The delay to the election, already postponed repeatedly since 2016, will anger supporters of the DRC’s fractured opposition and dismay observers who hoped it would bring a measure of security to the country. It is also likely to raise tensions and could prompt significant protests.

Corneille Nangaa, the head of the electoral commission, said officials have found enough voting machines for Kinshasa but had to get 5 million new ballots printed. Nangaa called on the country of some 40 million voters for calm.

The DRC’s outgoing president, Joseph Kabila, refused to leave office at the end of his second term in 2016 and only reluctantly agreed not to stand this time round. The country’s constitution limits presidents to two consecutive terms.

Emmanuel Ramazani Shadary, a former interior minister and Kabila loyalist, is standing for the ruling coalition instead.

Kabila has been in power since 2001 and the election would be the DRC’s first democratic transition of power since independence from Belgium in 1960.

Opposition candidates have warned any delay would be unacceptable.

“The CENI [election commission] president said there will be an election, rain or shine, on 23 December,” said Martin Fayulu, one of the two leading opposition candidates. “We cannot accept a change of Mr Nangaa’s position today.”

Fayulu is part of an opposition coalition, which issued a statement on Wednesday saying authorities “had ample time to prepare credible and peaceful elections”.

“They have not done so … so we are discovering their real objective: remain in power to pillage the country and kill the Congolese population which deeply hopes for a change,” the coalition added.

There were reports of protests by students at the University of Kinshasa after news of the delay became known.

Preparations for the repeatedly postponed vote were disrupted by last week’s fire in Kinshasa.

Many in the DRC believe the blaze was an attempt by authorities to smear the opposition and provide an excuse for a delay to the polls.

Local media reported CENI had cited three reasons for the delay: the deaths of more than 100 people in ethnic violence in the north-west this week, an outbreak of Ebola in the east and a shortfall in the number of ballot papers it had been able to distribute.

The DRC refused international assistance with the organisation of the election. This was despite the massive logistical challenge of a poll in a violent, unstable country the size of western Europe that has no proper road or rail system and a population of about 80 million people.

Campaigning has already been banned in Kinshasa because of concerns about violence.

Seven people have been killed by security forces during the campaign, while many more have been arrested.

The prosecutor of the international criminal court, Fatou Bensouda, said the risk of escalating violence could lead to the “commission of grave crimes”.

“In such a case, my office will not hesitate to take action … I wish to reiterate my appeal to the Congolese people, and more particularly to the authorities, political actors, their supporters and sympathisers, to do their utmost to prevent and avoid any conduct of criminal violence contrary to the Rome statute [pdf], anywhere and at any time. Violence is not an option,” she said.