Partial results indicate that Bolivia's President Evo Morales has lost a referendum to allow him to stand for a fourth term in office.
With 82% of the votes counted, those opposed to the proposed constitutional amendment are leading by 8.4 percentage points.
Mr Morales is Bolivia's first head of state of indigenous origin.
He said he would respect the result but accused right-wing sectors of waging a "dirty war".
An indigenous Aymara and former coca leaf producer, Mr Morales took office in January 2006. His current term ends in 2020.
The constitutional change would have let Mr Morales run for re-election in 2019 and potentially remain in power until 2025.
Opposition supporters have welcomed the partial results and gathered to celebrate in several Bolivian cities.
But the government still says there is "technical tie". It says rural votes, which usually arrive later to count centres, can make the difference for the 'Yes' campaign.
"We need to wait with serenity for the final results. It's not time to begin partying yet," said Mr Morales.
"Life goes on and our struggle and efforts [to improve the country] will continue", he added.
He is still a popular leader and the economy has grown steadily over the past decade, reports the BBC World Service Americas Editor, Leonardo Rocha.
However, many thought Evo Morales should not be allowed to serve 19 consecutive years as president, our editor adds.
Opposition leader Samuel Doria Medina urged Mr Morales to "recognise the results" and focus on solving Bolivia's problems in his remaining time in office instead of trying to run for another term.
Evo Morales in office
- First elected president in 2005: Began by renationalising the country's oil and gas industries and boosting social spending. Won a referendum in August 2008 on whether he should stay in office, and then a few months later a referendum approved his plans for a new constitution
- Re-elected in 2009: His second term followed a landslide win, and Mr Morales continued to pursue left-wing policies
- Again re-elected in 2014: He was able to run again despite the 2009 constitution limiting presidents to two consecutive terms in office. The Constitutional Court ruled his first term should not count because it had not taken place under the new constitution. His current terms ends in 2020
- Another run in 2019? A win in Sunday's referendum would let him stand again, and potentially serve until 2025, but is looking unlikely
'Charismatic and powerful'
Even if he loses the referendum, President Morales will have plenty of time before the next election to pick a successor and otherwise influence Bolivia's future, Michael Shifter, head of the US-based Inter-American Dialogue think tank, told Reuters news agency.
"He is one of the most charismatic and powerful leaders in Bolivian history. It is unlikely he is going to just retire from politics," he said.
"But perhaps for the first time in a decade, it is possible to imagine a Bolivia without Evo that does not return to the old times of economic and racial exclusion."
Despite a drop in the international price of oil and natural gas, the Bolivian economy has performed well in the past 10 years, growing on average 5% a year.
The government's socialist policies have also been successful in reducing extreme poverty.
But recent allegations that Mr Morales used his influence to favour a Chinese construction firm in Bolivia have damaged his approval ratings.
A former girlfriend of Mr Morales, Gabriela Zapata, holds an important position in the company, CAMC, which has secured more than $500m (£350m) in contracts with the Bolivian government.
Mr Morales rejected the allegations and said he had nothing to hide. He ordered an investigation into how the contracts were awarded.