Bolivians furious over what they saw as an attempt by leftist President Evo Morales to rig Sunday's election protested in the capital on Tuesday.
The demonstration, one of the largest in Bolivia in decades, was held as the country's electoral board was processing remaining ballots.
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.
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.
Suspicions of vote manipulation were sparked on Sunday after the official electoral board abruptly halted an electronic quick count showing Morales and Mesa were likely headed to a runoff.
EU calls for probe
On Tuesday, the EU expressed concern about the "unexpected interruption" of vote counting in the first round of elections in Bolivia.
Three preliminary vote counts — including one by the state Supreme Electoral Tribunal (TSE) — showed President Evo Morales would likely be forced into a head-to-head second round with runner-up Carlos Mesa.
'Serious concerns must be swiftly addressed'
"The unexpected interruption of the electronic vote-counting after the first round of the general elections in Bolivia has sparked serious concerns that need to be fully and swiftly addressed," said the EU in a statement.
"The Bolivian authorities, in particular, the Supreme Electoral Court, are expected to ensure maximum transparency of the counting and tabulation procedures and their outcomes. This is vital to guarantee the credibility of the electoral process, secure the confidence of the voters and respect the will of the Bolivian people. Recent incidents need to be investigated and we furthermore call on all parties to refrain from violence.
"The European Union has been a major partner of Bolivia in promoting economic and social development, as well as the strengthening of the rule of law. Our cooperation is founded on a shared commitment to democratic values and principles that should be upheld at all times."
Bolivians, furious over the results amid allegations of vote-rigging, set fire to electoral offices in three cities in the provinces late on Monday, including one in the city of Potosi, where two people jumped from a burning building to escape the flames.
To avoid a runoff election, Morales needed 40% of the vote and a 10-point lead over the runner-up.
Videos broadcast on social media and local TV also showed protesters pulling down a statue of late Venezuelan President Hugo Chavez in the city of Riberalta, and pitched battles between police and masked youths amid plumes of tear gas.
As protesters scuffled with police in the streets of the capital, La Paz, Mesa slammed the results as "shameful" and said he would not recognise them.
"We trust citizens won't accept this," Mesa told journalists in the lowland city of Santa Cruz. "This government has created an impossible situation. It's mocking the popular vote."
Just a day earlier, Mesa had celebrated making it to a second-round after an official count of nearly 84% of ballots showed Morales short of the votes needed to avoid a run-off.
Other polls also showed a tight race leading to a second round. Morales, who has won his previous three terms with solid majorities, however, insisted late on Sunday that he would get enough votes from rural areas for an outright win.
The uncertainty sparked protests in the landlocked South American country and fears among international election observers and diplomats about potential manipulation of the vote, with some worried it could trigger the kind of violent unrest that has recently roiled Chile and Ecuador.
"The result is in question and we're seeing the start of a conflict, a climate could be unleashed that moves this from tension to convulsion," said Bolivian political analyst Franklin Pareja.
Before the electoral court resumed updating of preliminary results on Monday, thousands of Bolivians protested outside a hotel in La Paz where the TSE had gathered to process votes.
Police fired tear gas at crowds, amid reports of clashes in protests elsewhere.
The TSE's earlier decision to halt the provisionary count with just 83.76% of the ballot completed had prompted the official monitor, the Organization of American States (OAS), to demand to know why.
"Those numbers clearly indicated a second round, a trend that coincided with the only authorized rapid count and statistics from the observation mission," Manuel Gonzalez, the head of the mission, said.
Gonzalez said the change in one day was "inexplicable" and a blow to voters' trust in the electoral process, but his comments fell short of calling the results illegitimate.
Argentina, Brazil and the White House also expressed concern over the interruption of the official reporting of results.
"Electoral authorities should immediately restore credibility and transparency to the process so that the will of the Bolivian people is respected," Michael Kozak, the US assistant secretary of state for western hemisphere affairs, said on Twitter.
If Morales wins, Latin America's longest continuous-serving standing leader would extend his rule to 19 years.