Left-winger Luisa Gonzalez is leading in Ecuador's presidential election overshadowed by the assassination of one of the candidates.
With nearly 80% of the votes counted, electoral officials say she has 33%, while her closest rival Daniel Noboa, a businessman, is on 24%.
The top two candidates will now go into a run-off on 15 October.
The poll was combined with a referendum, in which voters chose to end oil drilling in the Amazon.
The decision, supported by nearly 60% of those who voted, means the state-owned oil company will have to stop its operations in a block of Yasuní National Park, one of the world's largest biodiversity hotspots.
The area is home to hundreds of species of birds, amphibians and reptiles as well as indigenous people like the Tagaeri and Taromenani - who live in self-isolation.
The outcome is a significant blow to outgoing President Guillermo Lasso, who argued revenues from oil drilling were crucial for Ecuador's economy.
Some 100,000 police and soldiers were deployed to protect Sunday's first round of voting.
The snap election was called after Mr Lasso - a conservative former banker - dissolved parliament to avoid impeachment.
Sunday's voting was peaceful, much to the relief of Ecuadoreans fearful of the political violence that has taken hold of the country.
However, there were several shooting incidents in the run-up to the vote.
The new president will take office on 26 October and will serve only 18 months - the remainder of Mr Lasso's term.
Ms Gonzalez, a 45-year-old protégé of leftist ex-President Rafael Correa, was seen as the firm favourite of the eight politicians vying for the presidency.
But the assassination of candidate Fernando Villavicencio on 9 August in the capital, Quito, made the election difficult to predict.
Ms Gonzalez's promises of a return of generous social programmes appeal to Ecuadoreans hit hard by an economic crisis.
However, voters are divided politically.
Mr Correa still looms large in the country: he cut poverty while in power, but was then mired in corruption scandals and is now in exile in Belgium.
Those who want an end to his influence in Ecuador will back pro-business candidate Daniel Noboa, aged 35.
The only thing that unites Ecuadoreans is their need for peace and security. Everyone is hoping for a peaceful campaign ahead of the run-off.
Mr Villavicencio, 59, was an outspoken journalist who had uncovered corruption and denounced links between organised crime and officials.
Six men have been arrested in connection with his assassination, all of them Colombian citizens.
By Katy Watson in Quito & Jaroslav Lukiv in London