LA PAZ - Bolivia braced for a general strike on Tuesday hours after violence broke out in several cities when the main opposition candidate rejected presidential election results that seemed set to hand a controversial victory to long-time incumbent Evo Morales.

Opposition supporters reacted with fury, torching electoral offices in the southwestern cities of Sucre and Potosi, while rival supporters clashed in the capital La Paz.

Incidents were reported in cities across the South American country.

Carlos Mesa, who came a close second to Morales in Sunday's polls - forcing a run-off, according to preliminary results - denounced revised results released by election authorities as a "fraud."

"We are not going to recognize those results that are part of a shameful, consumated fraud, that is putting Bolivian society in a situation of unnecessary tension," said Mesa.

International monitors from the Organization of American States voiced "deep concern" at sudden changes to the election count to show Morales closing in on an outright victory in the first round.

Preliminary results released late Sunday showed neither Morales, 59, nor 66-year-old Mesa with a majority and "clearly indicated a second round," the OAS mission said.

The partial results put Morales in the lead with 45 percent of the votes, with Mesa on 38 percent, meaning Morales would have to contest a run-off for the first time.

But results released late Monday, after a long and unexplained delay, showed Morales edging towards an outright victory with 95 percent of the votes counted.

Mesa, a former president of the country between 2001-2005, accused Morales of colluding with the Supreme Electoral Tribunal (TSE) to tweak delayed results and avoid a run-off.

Opposition call general strike

The call for a general strike was issued by Fernando Camacho, head of an influential civil society organization in Bolivia's biggest city, Santa Cruz, where transport and businesses were expected to shut down from noon.

"Tomorrow we start at 12:00 to block this country," Camacho told opposition demonstrators late Monday, before holding talks with leaders from other regions.

Long lines formed at gas stations amid fears of shortages.

Riot-police dispersed a crowd who tried to storm the electoral offices in the Andean city of Oruro, south of La Paz.

Clashes were also reported in Tarija in the south, Cochabamba in the center and Cobija in the north.

'Subverting democracy'

The United States' top diplomat for Latin America said the Electoral Tribunal was attempting "to subvert Bolivia's democracy by delaying the vote count and taking actions that undermine the credibility of Bolivia's elections."

"We call on the TSE to immediately act to restore credibility in the vote counting process," the official, Michael Kozak, said on Twitter.

The OAS observer mission in the country expressed "surprise at the drastic and hard-to-explain change in the trend of the preliminary results revealed after the closing of the polls," it said in a statement.

It urged the election authority to "firmly defend the will of the Bolivian people" and called for calm on the streets.

"It is extremely important that calm is maintained and any form of violence is avoided in this delicate situation."

Longest serving president

Morales, Latin America's longest-serving president, is controversially seeking a fourth term.

He obtained Constitutional Court permission in 2017 to run again for president even though the constitution allows only two consecutive terms.

The former coca farmer and leftist union leader has led the poor but resource-rich Latin American country for the past 13 years, though his popularity has waned amid allegations of corruption and authoritarianism.

He has led the country since taking office in 2006, when he became its first indigenous president.

A new mandate would keep him in power until 2025.

As leader of his Movement for Socialism Party (MAS), Morales points to a decade of economic stability and considerable industrialization as his achievements, while insisting he has brought "dignity" to Bolivia's indigenous population, the largest in Latin America.

He has come under severe criticism this year as wildfires in August and September ravaged Bolivia's forests and grasslands, with activists saying his policies encouraged the use of blazes to clear farmland.