Bolivia’s president, Evo Morales, has accused opposition leaders and foreign powers of attempting a “coup” against him amid growing tensions over the result of Sunday’s desperately tight election.

In an angry televised speech on Wednesday, Morales said: “A coup d’etat is under way. The right wing prepared the coup with international support.”

Morales went into elections needing 40% of votes and a 10-point margin of victory to avoid a second-round runner against the main opposition candidate, Carlos Mesa. By Wednesday afternoon 97% of the official results had been processed, giving him 46.49% and a 9.5-point lead.

With most outstanding votes from remote rural areas expected to go in his favour, Morales repeated his declaration of a first-round victory, which he had made prematurely on Sunday night.

But on Wednesday the Organization of American States (OAS) said that a runoff should be held even if Morales breached the 10-point margin.

“In the case that … the margin of difference exceeds 10%, it is statistically reasonable to conclude that it will be by negligible margin,” said Manuel González, the head of the OAS election observation team in Bolivia. “Given the context and the problematic issues in this electoral process the best option continues to be the convening of a second round.”

International observers have expressed concern over an unexplained daylong gap in the reporting of results which was followed by a surge in Morales votes when the count resumed on Monday.

“Why did the government shut down the reporting of results?” asked Carlos Trujillo, US ambassador to the OAS, at a special session convened to discuss the Bolivian situation. “The government allowed a somewhat fair election because they did not realise their own popularity and thought they could win under their system. When they realised they could not win in the first round they shut down the results so that they could steal the election.”

The vice-president of Bolivia’s electoral board resigned on Tuesday, saying that the decision of the board’s six-member panel to suspend reporting results had discredited “the entire electoral process, causing unnecessary social convulsion”.

Mesa has accused Morales of trying to conduct “a giant fraud” and vowed that his party “will not recognize a fraudulent result”.

In a video statement on Wednesday, Mesa called for “permanent protests” until a second-round vote was confirmed, and said he would present evidence of electoral fraud.

Allegations of electoral fraud have already sparked street violence, in which anti-government protesters clashed with police, and set fire to electoral offices in eight of Bolivia’s regional capital cities.

On Tuesday the OAS said it would conduct an analysis of the election, focusing on the results reporting systems and the chain of custody of ballot boxes. However, the results of such an analysis are unlikely to please either side as the positions become increasingly entrenched.

Civil society groups in eight of the country’s nine departments called for a general strike that could bring the country to a standstill. “Not even an ant will move in Santa Cruz,” declared Luis Fernando Camacho, the leader of the civil society group for Santa Cruz, the country’s largest and richest city.

Morales has overseen relative stability and growth, but angered many by running for a fourth consecutive term despite a 2016 referendum which ruled against lifting term limits.

The results reflect the split between Bolivia’s urban population – which broadly backed the opposition – and the rural Andean populations that remain loyal to Morales, a former coca farmer.

“I don’t think Evo will accept the OAS’s calls for a second round,” says Jorge Derpic, a Bolivia specialist and assistant professor at the University of Georgia. “This is the first time we have seen protests by the middle classes in all the country’s major cities against Morales. Evo has called to mobilize his base – the coca growers, the miners and the campesinos [the rural poor] – and we could see further partisan violent clashes between rural and urban areas.”