(Adds comment from opposition)
By Monica Machicao and Daniel Ramos
LA PAZ, Oct 22 (Reuters) - Bolivians furious over what they saw as an attempt by leftist President Evo Morales to rig Sunday's election protested outside the hotel in the capital city of La Paz where the country's electoral board was processing remaining ballots on Tuesday.
With the official vote count at 97%, Morales extended his lead over his chief rival Carlos Mesa to 9.42 percentage points, just short of the 10-point lead he needs to avert a riskier run-off with Mesa.
Even if the pace of Morales' lead holds and he secures an outright win, the election's legitimacy has been scarred, with Mesa and his supporters vowing not to recognize that result.
Suspicions of vote manipulation were sparked on Sunday after the official electoral board, Supreme Electoral Tribunal (TSE), abruptly halted an electronic quick count after it showed Morales and Mesa were likely headed to second-round, with 84% of votes tallied.
When the quick count resumed amid an outcry on Monday, Morales had eked out a 10-point lead, sparking criticism from international election observers and a night of rioting across Bolivia, with several electoral offices attacked or set on fire, forcing two people to jump from a burning building in the city of Potosi.
Morales' government has denied any meddling and has called for calm. But in La Paz and other cities, protests resumed for a second day by nightfall on Tuesday.
"They robbed my vote," said Steve Quintela, a 31-year-old lawyer as he headed to downtown La Paz. "Of course the vote has been manipulated by the presidency."
Shouting insults at Morales and chanting "We're not afraid, damn it!" anti-government protesters filled avenues of the highland capital, moving past police barriers as firecrackers set off to summon more people to the demonstration rang out.
The demonstration was one of the largest in Bolivia in decades, according to a Reuters witness, who put the number of protesters at more than 100,000 people.
Mesa made a surprise appearance at the protest in front of the hotel after returning from Bolivia's second largest city of Santa Cruz, a key base of his support.
"Right now, a few meters from us, an enormous fraud is being committed to make us think there won't be a second round vote," Mesa told crowds in reference to the electoral board. "They're lying to the country and turning their backs on your vote!"
Police fired tear gas to disperse crowds for more than an hour, with some protesters responding by throwing rocks.
FEARS OF PROLONGED UNREST
In La Paz earlier on Tuesday, roads near markets were clogged as residents loaded up on food while long lines formed at gas stations amid fears of prolonged turmoil or a curfew.
The unrest marked a major jolt for the land-locked country, which has had a long stretch of political stability under Morales, Bolivia's first indigenous president and Latin America's longest continuous-serving standing leader.
In an attempt to calm the uproar, Morales' government walked back the president's comments from Sunday, when he declared he had won the election and only needed rural votes to confirm another "historic, unprecedented" victory for his government.
Stressing that the quick count was only a non-binding preliminary tally, Foreign Minister Diego Pary invited the official election observer, the Organization of American States (OAS), to audit the ongoing official vote count.
On Monday, the OAS said the change in the results of the quick count after the interruption had "drastically modified the fate of the election" and hurt confidence in the process.
"Whatever the result may be, we as the government are going to accept it," Pary told a news conference, noting the opposition had cried foul play before the vote began.
The president of the TSE, Maria Eugenia Choque, denied any attempt at electoral fraud in tearful comments before reporters.
But in a major blow to the board's credibility, its vice president, Antonio Costas, resigned in protest, saying the pause in reporting the quick tally had discredited "the entire electoral process, causing an unnecessary social convulsion."
Before his resignation, Costas told Reuters on Monday that he had not come under any political pressure to halt the tally but said he could not speak for the rest of the board.
The winner in the crowded race of nine candidates needs more than 50% of the vote, or 40% plus a 10-point lead. Morales had 46% and Mesa 37% in the legally-binding tally late on Tuesday.
Morales, who has stayed out of the public eye since Sunday, was scheduled to give a news conference early on Wednesday, when the OAS will meet to discuss the Bolivian election and pro- and anti-government demonstrators plan to mobilize.
A spokeswoman for Mesa's campaign, Cecilia Requena, called the plan for an OAS audit "good news" but added that protests must continue to keep pressure on the government to accept a second-round election.
"The audit will be more like an autopsy," Requena told Reuters. "We have to keep fighting and pressuring while there's still a margin." (Reporting by Monica Machicao and Daniel Ramos Additional reporting and Writing by Mitra Taj; Editing by David Gregorio, Leslie Adler & Simon Cameron-Moore)
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