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.
With the official vote count at 97 per cent, Morales extended his lead over his chief rival Carlos Mesa to 9.42 percentage points on Tuesday, 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 recognise that result.
Suspicions of vote manipulation were sparked on Sunday after the official electoral board, Supreme Electoral Tribunal, abruptly halted an electronic quick count after it showed Morales and Mesa were likely headed to the second round, with 84 per cent 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.
Shouting insults at Morales and chanting "We're not afraid, damn it!", anti-government protesters filled avenues of the capital, moving past police barriers as firecrackers set off to summon more people to the demonstration rang out.
A Reuters witness put the number of protesters at more than 100,000 people.
Mesa made a surprise appearance in front of the hotel. "Right now, a few metres 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.
In La Paz earlier on Tuesday, roads near markets were clogged as residents loaded up on food while long lines formed at service stations amid fears of prolonged turmoil or a curfew.
The unrest marked a major jolt for the 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.
Stressing the quick count was only a non-binding preliminary tally, Foreign Minister Diego Pary invited the official election observer, the Organisation 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.
In another 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".
Morales, who has stayed out of the public eye since Sunday, was scheduled to give a news conference early on Wednesday.
Australian Associated Press