With 99.8 percent of the ballots counted, Macri had 64 percent of the votes in the runoff election to 36 percent for Sen. Daniel Filmus, who was hand-picked by President Cristina Fernandez to run against one of her strongest opponents.

In his victory speech, Macri called for institution-building and teamwork so that Argentina can accomplish goals that reach beyond any one political term. Without criticizing Fernandez by name, he was describing a way of governing that his supporters consider to be radically different from the rule-by-decree populism that Fernandez calls her "model."

"It's very important that as politicians we think of ourselves as one city and one nation, all under one law. Only by respecting the law can we achieve justice and equality," Macri said.

"I don't promise miracles, I can't achieve that, but I do promise work," he added.

As mayor of Argentina's capital, Macri has been a powerful counterpoint to the national government and considered running against Fernandez in this year's presidential election before deciding on a safer re-election bid.

Buenos Aires' 2.4 million voters account for nearly 9 percent of Argentina's voting population, so the city's election was being watched as a possible indicator of sympathies for the president just two weeks before the Aug. 14 presidential primary. Still, the capital's voters often pick mayors from parties that oppose the president.

The big question now is which presidential candidate will win Macri's support. The mayor said he would talk with all of them, including Fernandez, and added that her call to congratulate him "was truly an honor."

In his concession speech, Filmus said the governing Front for Victory party should analyze why its message didn't persuade a majority of the city's voters, but insisted that coming in second gives them the responsibility to keep fighting for its priorities in the capital and beyond.

"We're going to keep going in the same direction," Filmus said, urging supporters to "work hard so that everyone lives better, with more equality and justice."

Macri used his presidency of the popular Boca Juniors football club as a springboard to enter politics as leader of the center-right PRO party.

His victory Sunday was the second blow in as many weeks to Fernandez's party. On July 24, the president's candidate for governor of Santa Fe province came in third behind a socialist and a comedian who backed by Macri.

While voting is mandatory in Argentina, only 72 percent of the city's voters cast ballots Sunday, the last day of the nation's winter school holiday. Of those who did vote, more than 91,000 who went to polling places despite particularly frigid temperatures cast blank or nullified ballots in protest of their choices.