There have been huge protests since the polls
Addressing tens of thousands of people on the streets of the capital, Antananarivo, Marc Ravalomanana said there would be an inauguration ceremony on Friday.
There has been no immediate response from the government.
Mr Ravalomanana claims to have been cheated out of victory in last December's poll, and has repeatedly called on President Didier Ratsiraka to step down.
Official results said that neither candidate had obtained the 50% of the votes required to be declared the winner.
Army 'neutral'
Mr Ravalomanana said he had received assurances from the military that they would remain neutral.
He said he had asked the army to keep the peace, but had not asked them to choose between him or President Ratsiraka.
Mr Ravalomanana's declaration came only a day after he agreed to take part in a second round of voting, monitored by international observers.
State radio announced on Wednesday that the run-off would take place on 24 March.
"I have listened to the calls of pastors and priests who told me to take power. I am ready to do it," Mr Ravalomanana told a huge crowd of supporters in the capital's May 13 Square.
He said: "Today we'll take [power], but we'll take it officially on Friday."
Failed talks
Talks brokered by the Organisation of African Unity (OAU) collapsed on Tuesday, but OAU Assistant Secretary General Said Djinnit said they would resume in the next few days.
Mr Ravalomanana said there would be no more international mediation.
In recent weeks, Antananarivo, where Mr Ravalomanana is mayor, has been the scene of massive street protests by up to 100,000 opposition supporters, while a general strike has paralysed the country.