The runner-up in Democratic Republic of the Congo’s presidential election says he in fact won the vote by a landslide, as rising violence across the country fuels fears of a wider law and order breakdown.

Martin Fayulu’s campaign team claimed it had evidence its candidate had scored more than 60% of the votes in the much-delayed election on 30 December, 42 points more than Félix Tshisekedi, who was declared the winner by the electoral commission early on Thursday.

In a speech to hundreds of supporters who gathered in the capital, Kinshasa, on Friday, Fayulu denounced what he called the “people’s stolen victory” and said he would file a challenge to the official results at the constitutional court on Saturday morning.

Fayulu’s figures are understood to be close to those compiled by the influential Catholic church, which deployed 40,000 observers on the day of the election.

The church has refused to reveal who won according to its findings, but diplomats briefed on the church data say it indicated a clear victory for Fayulu, in line with pre-election polls that had put him at least 20 points ahead of Tshisekedi.

The Fayulu camp figures are also very similar to those cited in hundreds of pages of documents leaked by a whistleblower and passed to the Guardian overnight on Thursday. The documents, which the Guardian has been unable to verify independently, are purported to be the electoral commission’s authentic count.

An electoral commission spokesperson denied that there was any difference between the officially published results – which gave Tshisekedi 38%, four points ahead of Fayulu – and any other figures compiled by the body. Experts said the results in the leaked documents were “plausible in parts” but expressed scepticism about the claimed margin of Fayulu’s apparent win and the turnout figures cited.

Supporters of Felix Tshisekedi celebrate outside the Union for Democracy and Social Progress headquarters in Kinshasa on Thursday. Photograph: John Wessels/AFP/Getty Images

At least nine people have been killed and many more injured since the announcement of the result on Thursday. Five police stations have been attacked and scores of protesters arrested in a series of clashes with security agencies. A prison breakout and a massacre by a militia have further intensified security concerns across the vast, strategically crucial central African state.

Most of the demonstrations have involved supporters of Fayulu, a former business executive, who immediately rejected the official results on Thursday, accusing Tshisekedi of doing a deal with outgoing president Joseph Kabila and calling for a campaign to resist what he called “an electoral coup”.

In comments to the UN security council via video link on Friday, the president of the electoral commission defended the vote’s credibility and attacked the church. “I’d be very interested to know what party they work for,” said Corneille Nangaa. “I challenge anyone to say they have the pretension to have collected all the vote tallies.”

The CENCO group of Catholic bishops told the security council it was independent and said the electoral commission should release its records to allow for verification and “dispel doubt among the population”.

Q&A Why is the DRC election so important? Show The Democratic Republic of the Congo's sheer size, its political history and its myriad problems are all reasons why observers have followed its election so closely. The vast, resource-rich country, with a population of 80 million spread over an area the size of western Europe, has never known a peaceful transition of power since its independence from Belgium in 1960. It remains one of the poorest places in the world, racked by war and disease and with massive inequality. In the east, where scores of militia commanders battle for control of mines, an outbreak of Ebola has killed more than 300. Countrywide an estimated 4.3 million people are displaced. It is still recovering from a civil war triggered by the fall of the dictator Mobutu Sese Seko in 1997, in which 4 million people died. Joseph Kabila has been in power as president since his father, Laurent-Désiré Kabila, the victor of that conflict, was shot dead in 2001. The country’s problems have been exacerbated by the refusal of Kabila to leave power after the end of his second mandate two years ago, which is why December’s much-delayed election was invested with so much hope and trepidation. Chaos at polling stations on the day of the vote dampened hopes that the election would bring a measure of political stability. The announcement by the electoral commission on 10 January that Felix Tshisekedi had won confounded polling that had put another opposition figure well in front and raised fears of a backroom deal ultimately keeping Kabila in power. Photograph: Stefan Kleinowitz/EPA

Fayulu’s claim of substantial victory will fuel a febrile atmosphere that threatens to tip DRC into a cycle of protest, violent repression and worsening insecurity – dashing hopes that the election would mark a turning point in the country’s troubled history. Polls in 2006 and 2011 – both won by Kabila – saw significant bloodshed.

Reports compiled by international NGOs and embassies seen by the Guardian reveal the extent of growing violence. Thousands of police and soldiers from the regular army as well as the feared republican guard, an elite unit loyal to Kabila, were deployed across the country as protests broke out.

In Kikwit, the capital of Kwilu, between three and six civilians were killed and two police officers badly beaten as soldiers used live firing to clear unidentified youths who had erected barricades on Thursday evening. Aid workers described the situation on Friday as very tense, with shots heard in several locations. Several arrests were made by police, including a journalist who was later released.

There are also reports of a prison breakout in the town, during which 90 convicts escaped and five were shot dead. Much of the prison was reportedly destroyed.

Supporters of Fayulu listen to his speech at a rally in Kinshasa on Friday. Photograph: Jérôme Delay/AP

In Mbandaka, a port town on the Congo river in the centre of the country, there were protests on Friday morning after the overnight deaths of two civilians, one a woman allegedly killed by a soldier. Soldiers were reported to have fired into the air to clear the crowds and a senior local leader of Fayulu’s coalition was arrested after calling for protests on a radio station.

In the major town of Kisangani, police and soldiers stormed the campus of the university and makeshift road blocks in the Makiso neighbourhood to control a “massive and violent situation” with sporadic shooting, according to aid workers. The local offices of the main ruling party were burned and a drinking club belonging to the town governor was looted. Local reports described calm on Friday but nervous inhabitants were said to be withdrawing money from banks.

Reinforcing fears of a more general breakdown of security, the bodies of seven civilians were discovered on Thursday morning in North Kivu, a restive eastern province. The find brings the death toll from a suspected militia attack earlier this week to 18, one of the most deadly for months, diplomatic sources told the Guardian.

Tshisekedi’s apparent victory surprised some observers who believed authorities would ensure victory for the government candidate, Emmanuel Ramazani Shadary, who was handpicked by Kabila as his successor.

Kabila, has ruled since the 2001 assassination of his father, Laurent Kabila, who overthrew the long-serving dictator Mobutu Sese Seko in 1997.