The head of an international body auditing Bolivia's disputed election results resigned unexpectedly on Friday, casting further uncertainty over a vote that sparked deadly riots and delivered President Evo Morales a fourth term. 

The chief of the technical mission from the Organisation of American States (OAS), Mexican Arturo Espinosa, announced he is stepping down from the role just a day after beginning the review of the controversial poll. 

"I have decided to withdraw from the audit so as not to compromise its impartiality. I should have informed the OAS about previous public statements (declarations) about the electoral process in Bolivia," he wrote in a tweet.

Espinosa wrote two articles related to Bolivia's elections for a Mexican news website in the past two weeks, including one - published after the election - which raised doubts over the poll's transparency. 

The 20 October election result, ratified on Friday by Bolivia's own Supreme Electoral Tribunal (TSE), saw Morales narrowly secure the 10-point gap needed to win outright - but only after an abrupt and unexplained shift in the vote count in his favor. 

Opposition candidate Carlos Mesa criticized the TSE's latest result calculations, saying that they show Morales committed a "fraud" and "an aggression against the good faith of the international community." 

The 66 year-old former president has also refused to take part in the OAS audit, calling instead for the results given by the electoral court (TSE) to be annulled as a precondition of his co-operation.

Riots broke out almost as soon as the election ended and Espinosa's resignation now threatens to heighten tensions. 

Protests erupted in various cities in Bolivia on Friday, especially in the south of the administrative capital La Paz, where roads were cut off and riot police guarded vital crossings. 

The extended clashes have seen 191 people detained and 60 police officers injured, said police chief Julio Cordero said on Friday. 

But some opposition groups protesting the election now support neither Morales nor Mesa.

Popular assemblies or "Cabildos" held on Thursday in La Paz and the eastern city of Santa Cruz have rejected the OAS audit and demanded new elections. 

The council of La Paz has even proclaimed "Neither Mesa, nor Evo Morales!", in favour of holding new elections without either of the two main candidates who stood on 20 October. 

The president said on Friday that Bolivians should wait for the OAS audit report, which should be ready in two weeks. 

Bolivia's first indigenous president, Morales has been in power since 2006. 

Already Latin America's longest-serving leader, Morales is looking to remain in power until 2025 with a fourth term. 

The country's constitution limits a president to two successive terms, and a 2016 referendum rejected a bid by Morales to remove term limits. 

But Bolivia's constitutional court authorized him to stand for a fourth mandate. 

The court, like the election tribunal, is made up of members appointed by Morales's Movement for Socialism.

(Cover: Security forces are seen during clashes with demonstrators taking part in a protest in La Paz, Bolivia, October 22, 2019. /VCG Photo)