An independent international audit of Bolivia's disputed election in October has detailed "deliberate" and "malicious" efforts to rig the vote in favour of then-President Evo Morales.
The Organization of American States (OAS) said the measures included hidden servers and falsified signatures.
Mr Morales fled to Mexico after the army urged him to quit. He has not commented on the report.
The interim president has pledged new elections but no date has yet been set.
Jeanine Áñez, a former right-wing senator, called the OAS findings "frightening". She has previously said Mr Morales, Bolivia's first indigenous president who was seeking a fourth term, should face prosecution if he returns to the country.
According to the OAS, the "intentional manipulation" and "serious irregularities" included "changes in the minutes and the falsification of the signatures of poll officials", making it impossible to validate the official results of the 20 October vote.
In the processing of the results, it said in the 95-page final report (in Spanish), the data was redirected to two hidden servers and not controlled by officials at the Supreme Electoral Tribunal, opening the way for the manipulation of data.
There was also "loss of sensitive material [and] a significant number of errors".
"Based on the overwhelming evidence found," the regional body said, "what can be affirmed is that there has been a series of intentional operations aimed at altering the will expressed at the polls."
A former coca farmer, Mr Morales took office in 2006 and won plaudits for fighting poverty and improving the economy. However he drew controversy by defying constitutional limits to run for a fourth term.
Questions about the legitimacy of the election were first raised when the results count inexplicably paused for 24 hours. The final result gave Mr Morales slightly more than the lead he needed to win outright, sparking protests across the country.
An initial OAS report had already pointed to "clear manipulation" of the election and called for it to be annulled. In response, Mr Morales agreed to replace the electoral authorities and hold a fresh poll.
But days later, on 10 November, Mr Morales stepped down and sought asylum in Mexico following an intervention by the chief of the armed forces calling for his resignation. He denounced the move as a "coup" .
Since leaving office, Mr Morales has been accused by the interim government of terrorism and sedition over his alleged role in fomenting unrest, allegations he disputes.
Reacting to the OAS final findings, Ms Áñez said: "This report... accurately describes the way in which the MAS [Mr Morales' political party] government weaved together a network of corruption from the powers of the state to mount this monumental fraud."
Speaking at a news conference, she added: "The government of Evo Morales had no scruples in having a laugh at the expense of Bolivians."
At least 30 people have died in violence since the election, most of them after Mr Morales' resignation. Last month, Congress approved legislation paving the way for a new vote barring Mr Morales from running.