With some 99 per cent of ballots counted, Alberto Fernandez, the main opposition candidate who has Kirchner as his running mate, took nearly 48 per cent of the vote to 32 per cent for Macri.
Voters in Argentina have cast doubt on President Mauricio Macri's chances of re-election by backing his opponent by a wide margin in a primary vote, early official results showed on Sunday.
"It is a shocking and practically irreversible result, which leaves the government in a power vacuum in the middle of a very delicate economy", said Shila Vilker, an analyst with Argentine firm Trespuntozero.
A person casts a vote at a polling station during the primary elections in Buenos Aires, Argentina, on August 11, 2019.
"Recognizing that we have had a bad election, that forces us, starting tomorrow, to redouble our efforts so that in October we will get the support that is needed to continue the change", Macri said. Visit MarketWatch.com for more information on this news.
Macri conceded defeat on Sunday even before the release of official numbers, which were delayed by the slow counting of votes in the province of Buenos Aires.
Mauricio Macri on August 11.
The former president nationalised pension funds, imposed currency controls and tampered with economic statistics during her presidency between 2007 and 2015.
Re-electing Mr. Macri would mean seeing through painful cuts in public spending as part of the $75-billion standby agreement he negotiated with the International Monetary Fund a year ago.
Centre-left nominee Alberto Fernandez led by around 15 points after partial results were revealed as centre-right, pro-business Macri admitted it had been "a bad election". "We always fix the problems that others create", said Fernandez to thousands of cheering supporters in Buenos Aires on Sunday night.
"We were never insane while in power", he told supporters in Buenos Aires.
Monday's likely sell-off will probably be exacerbated by a last-minute wave of optimism about Macri's prospects that caused the Merval stock index to jump 6.7 pecent. The Argentine currency will start trading around 9 a.m.
Fernandez can also avoid a run-off if he wins with 40% with a lead of at least 10 percentage points over his closest challengers.
"It's one in another line of elections globally where the pollsters had absolutely no clue", said Walter Stoeppelwerth, chief investment officer at Portfolio Personal Inversiones, a broker in Buenos Aires.