With 96 percent of all polling stations counted, Andry Rajoelina was on 39.38 percent and Marc Ravalomanana on 34.19 percent -- pointing towards a close race for the presidency in the head-to-head second round.
Outgoing president Hery Rajaonarimampianina came in third place with about nine percent.
Thirty-six candidates participated in the first-round election.
The second-round duel between the two top vote winners -- required if no candidate scores above 50 percent in the first round -- is to take place on December 19.
The three leading candidates have all raised allegations of fraud and malpractice by election authorities.
"The electoral process is at a delicate moment, sensitive to any tensions and rivalries, so all stakeholders are urged to protect the best interests of the nation and to guarantee order," the police said in a statement.
Both Ravalomanana and Rajoelina -- fierce rivals -- were banned from contesting the last election in 2013 under international pressure to avoid a repeat of political violence that engulfed the island in 2009.
Ravalomanana ruled from 2002 to 2009 until he was ousted in a military-backed coup that installed Rajoelina, who held power until 2014.
Rajaonarimampianina succeeded him, ruling until earlier this year.
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