Marc Ravalomanana, the former Madagascar president who has sought another term, urged supporters on Sunday to protest as he appeared headed for defeat in a run-off poll.
“I call on all Madagascans who feel they have suffered injustice and a violation of their right to vote to stand up and dare to defend their choice,” he said in a speech.
“Our country is sacred, God is with us, fear not, just believe,” the candidate added.
His rival and another former president, Andry Rajoelina, was credited with victory according to provisional results from the vote held last Wednesday.
The two-round election was beset by allegations of fraud from both sides and a contested result would raise the risk of continued political instability in the coup-prone Indian Ocean island nation.
The result published on Sunday by the electoral commission after more than four million ballots were counted gave Rajoelina 55.08% and Ravalomanana 44.92% of the vote, with more than three-quarters of voting stations reporting in.
Just under five million people are estimated to have gone to the polls, and a final tally is expected in the coming days.
Both candidates had claimed victory after polls closed on Wednesday, and EU election observers have said they had not seen evidence of wrongdoing.
“The Madagascans voted in a peaceful atmosphere in a transparent and well-organised poll,” mission head Cristian Preda told reporters.
Rajoelina and Ravalomanana have fought a fiercely personal duel for power, and both spent lavishly on campaigning with promises and handouts distributed liberally to voters who are among the poorest in Africa.
Ravalomanana, 69, is a former milkman from a peasant family who built a business empire.
Elected president in 2002, he was forced to resign seven years later following protests fueled by Rajoelina, who had been elected mayor of the capital Antananarivo.
Rajoelina, now 44, was then installed by the army and ruled until 2014. He is a former events planner and successful entrepreneur with slick communication skills.