Ruling party candidate Daniel Scioli, Fernandez's chosen successor, conceded late yesterday and said he had called Macri to congratulate him on a victory that promises to chart Argentina on a more free market, less state interventionist course.
"Today is a historic day," said Macri, addressing thousands of cheering supporters as horns were heard blaring across Buenos Aires. "It's the change of an era."
The victory by the business-friendly Macri comes after he did better than expected in the first round on October 25, forcing a runoff with Scioli, the governor of the vast Buenos Aires province.
Macri campaigned on promises to reform and jumpstart the South American country's sagging economy. He pledged to lead by "listening more and speaking less" than Fernandez, something he frequently said on the campaign trail.
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"I'm so happy," said Julia Juarez, a 66-year-old retired teacher who was one of thousands watching the returns at Macri's bunker. "Argentines are tired of this government. Tired of the corruption. We are ready for something new."
He said a Macri win would subject this nation of 41 million people to the market-driven policies of the 1990s, a period of deregulation that many Argentines believe set the stage for the financial meltdown of 2001-2002.
Macri's win signals a clear end to the era of Fernandez, who along with her late husband and predecessor, Nestor Kirchner, rewrote the country's social contract, gaining both rabid followers and fierce critics along the way.
The power couple spent heavily on programmes for the poor, raised tariffs to protect local economies and passed several progressive laws, including the legalisation of gay marriage in 2010.
Macri frequently repelled Scioli's claim that Macri represented policies of the past, saying he would lead with "21st century development" as opposed to "21st century socialism", a term used by supporters of the late Venezuelan President Hugo Chavez and his successor, Nicolas Maduro.