Scioli, expected to follow in the footsteps of Peronist leader President Cristina Fernandez de Kirchner, had in recent weeks been polling with a wide lead.
But Buenos Aires mayor Macri, who has pitched business-friendlier approach to help address the country's ailing economy, surprised by driving a nail-biting race that will be decided in a 22 November contest.
Victory Front candidate Scioli drew 36.8%
