Bolivia's centrist presidential candidate Carlos Mesa concedes he will head the country's opposition, with ex-president Evo Morales' leftist MAS party appearing to have taken a landslide victory in Sunday's elections.
Two independent surveys have given MAS candidate Luis Arce more than 50 per cent of the vote, a result which would allow him to avoid a run-off against Mesa, whom the surveys gave about 30 per cent.
Officially, only around 20 per cent of the votes have been counted so far.
But the MAS claimed a victory on Monday after the survey results were published overnight, in what would be a spectacular comeback for the weakened party that governed the country for 13 years.
Mesa said his party "assumes that the result given yesterday is a result that clearly defines a winner in the first round."
UN Secretary General Antonio Guterres congratulated Bolivians on holding "highly participative and peaceful" elections and encouraged political and social leaders to "work together" for democracy, human rights and "national reconciliation" in the deeply divided country.
The MAS holds a majority in the outgoing Congress but it was left reeling after after fraud allegations against Morales sparked violence and led to elections held a year ago being voided.
The Andean country's first indigenous president went into exile and Anez, a right-wing senator, took power as interim president.
Her government faced accusations of a poor handling of the coronavirus pandemic, corruption and repression of opponents, which contributed to many voters preferring the left, according to analysts.
"It shouldn't be that surprising that Arce would secure such a commanding victory," said Mark Weisbrot, co-director of the US think tank Center for Economic and Policy Research.
"When he was economy minister under Evo Morales, Bolivia experienced impressive economic and social gains that benefited millions of people," he added.
The beneficiaries included indigenous people constituting nearly half of the population, while strong economic growth allowed Bolivia to reduce poverty by 42 per cent, Weisbrot said.
Morales, whom many now expect to return from exile in Argentina, said on Monday the MAS would resume its role in leading the region's economic growth.
"With (Arce) we shall, united, again lift Bolivia up," he tweeted.
Bolivia's next president is due to be sworn in between October 31 and November 14.
Australian Associated Press