Conservative Mauricio Macri wins Argentine presidency
With most votes counted, Mr Macri had more than 53% to Daniel Scioli’s 47%.
Loud cheers erupted at Mr Macri’s campaign headquarters after TV exit polls suggested he had won.
Mr Macri, the mayor of Buenos Aires, lost to Mr 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 Mr Scioli lost ground to his rival in the month since.
If Mr Macri’s apparent victory is confirmed, it will be the first time in more than a decade that Argentina’s centre-right opposition has wrested the presidency from the centre-left Peronists.
Mr Macri, the leader of the Cambiemos (Let’s Change) coalition, went into Sunday’s vote with a comfortable lead in opinion polls.
He campaigned on pledges to bring new investment into the ailing economy, tackle crime and fight corruption.
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, Mr Scioli had been expected to win by a greater margin in October.