Centre-left Michelle Bachelet held a big lead in Chile's presidential election but fell short of the 50% of votes necessary to avoid a run off, according to a media projection tonight.
Ms Bachelet received 45.7% of the vote, radio Bio-Bio said.
Her second-place rival, Evelyn Matthei, the candidate for the right-wing governing coalition, had 24.9%.
Red-hot favourite Ms Bachelet, who governed Chile from 2006 to 2010, is widely expected to win a potential December runoff.
The centre-left Bachelet had promised to tackle deep inequality by improving education, a need that has led to loud demonstrations in recent years.
She also promised to upend a constitution that dates back to Augusto Pinochet's 17-year dictatorship.
Political stalemate and Bachelet's own inexperience stymied her ability to push through all the reforms she wanted in her first term.
Those in her circle, however, think there is more room for manoeuvre this time.
All 120 lower house seats and 20 out of 38 Senate seats are also being contested today.
Under the Chilean system, the governing coalition needs more than a simple majority to pass some kinds of legislation.
Yet its electoral system tends to result in one person from each coalition being elected from each district, creating an effective tie in Congress - a Pinochet-era legacy that Bachelet wants to change.
Education reform would require her to control four-sevenths of both houses - at least 22 senators and 69 from the lower house. Analysts said this can be done.