Conservative Mauricio Macri has been confirmed as the winner in Argentina's presidential elections, as his ruling party opponent conceded.
With almost all votes counted, Macri had 52% to Daniel Scioli's 48%.
Loud cheers erupted at Macri's campaign headquarters after TV exit polls suggested he had won.
Macri, the mayor of Buenos Aires, lost to Scioli, who is the governor of Buenos Aires province, in the first round of voting in October.
But neither candidate managed to win the vote outright, forcing a run-off - the first in the country's history - and Scioli lost ground to his rival in the month since.
Macri's victory will be the first in more than a decade for Argentina's centre-right opposition.
He went into Sunday's vote with a comfortable lead in opinion polls, and campaigned on pledges to bring new investment into the ailing economy, tackle crime and fight corruption.
A month ago, Argentina looked set to elect Daniel Scioli and keep President Cristina Fernandez's party in power. But in the run-off vote on Sunday - the first in Argentina's history - voters opted for radical change, electing free-market candidate Mauricio Macri.
After a campaign marked by strong mutual attacks, tonight both Macri and Scioli gave measured, respectful speeches after results were announced - perhaps a sign that there can be a constructive dialogue in the coming few days.
Macri will have a lot on his plate in the coming days and after 10 December, when he takes office.
Markets will be watching how he ends currency controls - one of his key promises to get Argentina growing strongly again. He will also need to build a coalition in Congress, as he does not have the majority to rule.
The son of one of Argentina's richest men, he had a long career in business before entering politics.
A close ally of current President Cristina Fernandez de Kirchner, Scioli had been expected to win by a greater margin in October.
