Bolivian President Evo Morales said he was confident he would win Sunday's election outright, despite an official preliminary count of 84% of ballots that showed he would probably be forced to a run-off vote.
Morales, 59, said he was certain outstanding votes from rural areas, where he tends to have stronger support, would deliver him another "historic" victory and a congressional majority.
Morales needs at least 40% of votes and a 10-point lead over his closest rival to avoid a December 15 second-round vote with the runner-up.
The partial count by the electoral board showed Morales had won 45% of votes, against 38% for chief rival Carlos Mesa.
Mesa, who said earlier he did not trust the TSE, celebrated the results amid cheering supporters, saying: "We're in the second round!"
"Democracy is the most important value for which we are fighting," added Mesa, 66, driving home the theme of his campaign. "We're not going to lose it."
Morales is running in defiance of term limits and despite a 2016 referendum in which Bolivians voted against allowing him to seek a fourth consecutive term. A local court ruling allowed him to run anyway.
As he did in the 2014 election, Morales has promised to retire after the five-year term is over.
Chi Hyun Chung, the candidate of the right-wing Christian Democratic party, was running in third place with close to 9% of the vote, indicating his support base will be a key target for Morales and Mesa in the second round.
Morales, a former union leader for coca growers, has managed to hang onto power as most other leftist presidents in South America elected in the previous decade have since been succeeded by right-leaning leaders.
He has overseen a long stretch of political and economic stability for Bolivia, the continent's poorest country. But support for him has slipped amid slowing economic growth and concerns about government corruption and anti-democratic practices.
Whoever wins will likely have to govern without a majority in Congress and with a gloomier economic outlook, as the commodities-fueled boom that drove rapid economic growth in Bolivia in recent years has ended and the country's important natural gas reserves have dwindled.