Thousands of jubilant supporters of Fernández's running mate and new boss, Alberto Fernández, celebrated the nation's return to the center-left after conservative incumbent President Mauricio Macri conceded defeat in Sunday's election.

"We're coming back! We're coming back!" they chanted, waving sky-blue and white Argentine flags.

Alberto Fernández, who is no relation to the former president, had 48.1% of the votes compared to 40.4% for Macri, with almost 97% percent of the votes counted, election officials said. He needed 45% of the vote to avoid a runoff.

Alberto Fernández served as chief of staff from 2003 to 2007 for Cristina Fernández's late husband and predecessor as president, Nestor Kirchner. He remained in the position during part of Cristina Fernández's term as president but left after a conflict with farmers in 2008.