Business favorite Macri led the count with 36 per cent of the vote to 35 per cent for Scioli, with just more than two-thirds of polling stations reporting -- though most results from Scioli's stronghold, Buenos Aires province, had yet to arrive.
Under Argentine electoral law, in order to win outright, a candidate must claim more than 45 per cent of the vote, or at least 40 per cent with a margin of 10 points over the runner-up. The runoff will be the first in the country's history.