The fallout from Venezuela's disputed presidential election continued to spread this week as the government of Nicolás Maduro threatened to jail the opposition candidate and arrested an American filmmaker accused of working for US intelligence.

While electoral officials prepared a wider audit of the narrow vote on 14 April, opposition candidate Henrique Capriles added to the tension on Thursday by demanding scrutiny of registers containing voters' signatures and fingerprints.

Capriles has refused to accept the declared result, according to which he was defeated by less than 2%. Alleging thousands of cases of vote-rigging and other electoral law violations, he called on his supporters to stage peaceful cacerolazo – a popular form of protest where people bang on pots and pans.

The protests have been called off but the government said the demonstrations last week led to nine deaths, 78 people injured and the burning down of clinics and party headquarters. This too is disputed, but the ruling United Socialist party initiated an investigation in the national assembly this week into whether Capriles should be held responsible.

"The deaths ordered by the fascist murderer Capriles cannot go unpunished," Diosdado Cabello, the head of the national assembly noted on Twitter on Thursday. "The investigations are going forward."

The prisons minister, Iris Valera, said she had a cell waiting for the opposition leader. "Capriles is the intellectual author of these crimes and will not go unpunished," Varela said on state television. "The only good news for you is that the prison waiting for you, Capriles Radonski, is not like the ones we inherited from the previous governments."

Capriles said he was ready to go to jail rather than accept what he describes as a "stolen" election but he denied instigating violence.

"If they want to bring me to trial what's their reason?" Capriles said on Wednesday. "For asking that the vote boxes be opened? For asking people to bang pots and pans? If that's the cost then do it fast. Don't keep threatening."

Capriles spent 119 days in prison for his alleged involvement in a violent protest outside the Cuban embassy after a failed coup against former president Hugo Chávez in 2002.

The ruling camp has promised to audit the vote yet claimed the result is "irreversible". Capriles supporters say the audit will not be valid and will be boycotted by his movement unless it includes detailed information from voting notebooks as well as checks on whether people voted more than once and whether votes were registered in the names of dead people.

The ruling party and its supporters believe the unrest is the latest attempt by the United States to delegitimise a hostile government that is sitting on the world's biggest oil resources. The US has been reluctant to recognise Maduro as president and called for a recount.

Earlier on Thursday authorities detained a US citizen, Timothy Hallet Tracy, who they accuse of trying to destabilise the country on behalf of an unnamed US intelligence agency.

"We detected the presence of an American who began developing close relations with these [students]," said the interior minister, Miguel Rodriguez, in a press conference. "His actions clearly show training as an intelligence agent, there can be no doubt about it. He knows how to work in clandestine operations."

Rodriguez said Tracy, 35, from Michigan, had received financing from a foreign non-profit organisation and had redirected those funds toward student organisations.

The ultimate aim was to provoke "civil war", he said.

Friends and family of Tracy told the Associated Press that he had been in Venezuela since last year making a documentary. "They don't have CIA in custody. They don't have a journalist in custody. They have a kid with a camera," said Aengus James, a friend and associate of Tracy's in Hollywood, California, told the agency.

In Washington, state department spokesman William Ostick said US consular officials in Venezuela were attempting to meet and speak with Tracy. He rejected Maduro's repeated allegations that the US is attempting to undermine Venezuela's government.

Whether the authorities arrest Capriles and risk creating a martyr remains to be seen, but the opposition leader is not the only focus of government efforts to restore authority in the wake of a result that shocked many in the ruling camp, not least because Maduro lost many of the urban districts where his predecessor Hugo Chávez had been dominant.

About 270 people were detained during the protests, including 195 students and juveniles. Opposition provocateurs have been accused of firebombing neighbourhood health clinics staffed by Cuban doctors, but several of the alleged arson attacks have susbsequently been disproved.

"We have no reports of burnt centres. We saw some aggression but no destruction. I think the government exaggerated this to create a mood or opinion that favoured them", said Maria Esperanza Hermida of Provea, a human rights watchdog.

NGO monitoring groups also dispute the number of casualties from the violence, saying the government included several victims of apparently random street crime, while ignoring other political killing cases in which opposition supporters were the victims.

"The government is not doing this investigation with the thoroughness that it calls for … we find this is being handled politically and not with the transparency it requires," said Marcos Ponce of the Observatory of Social Conflict, a civil rights NGO.

Juan Jose Faria, a journalist with La Verdad in Maracaibo, in western Venezuela, was taken to jail along with his crew for covering the protests. Faria, a crime reporter, had gone to the San Francisco neighborhood to verify several claims of police brutality, and was detained by a sergeant who blocked their car and surrounded them accompanied by a group of 20 civilian men on motorcycles.

He spent close to five hours in a detention cell and was transfered to another building where he was freed with no further charges.

Faria and his team were stripped and identified as criminals and all of their cameras and notebooks were taken away.

"The man who detained us told us there had been a coup and accused of being fascists trying to destabilise the government. I thoght we would be killed because we all know that when there is a coup you are stripped of your constitutional guarantees and anything can happen", Faria said.

The opposition has claimed that state employees have been threatened, punished or fired for joining opposition protests or failing to show sufficient support for Maduro.

The minister of labour, Maria Cristina Iglesia, said this was a lie in an interview with a state-run radio station.

"This could be part of a larger montage to cover the incidents of violence that took place in our country, and that were promoted by fascism: it is a way of covering up the events," Iglesias said. "It is a very small minority that has the venom of hate, of fascism and that wants to transmit it to others".