Ravalomanana claims to have won the last election
He issued a "call to the Malagasy people" to fight those blocking what he believes is his rightful claim to lead the country.
Mr Ravalomanana controls the capital, Antananarivo, but supporters of incumbent President Didier Ratsiraka have established a rival power-base in the port city of Tamatave and imposed an economic blockade.
Mr Ravalomanana's rallying cry - made in a televised address - marks a dramatic change of tone after months of generally peaceful demonstrations and political stand-off.
'Enemies of the nation'
"People of bad intent have declared war on the Madagascan nation with the help of a few military men," he said.
"They are starving the people by erecting roadblocks and destroying bridges.
"We declare that these people are enemies of the nation. Henceforth, we're going to track them down wherever they are."
Mr Ravalomanana urged the army to help lift the blockade and to rebuild bridges which have been attacked in areas including Fianarantsoa.
He urged active and reserve members of the security forces to join a "national security council".
Flanked by members of his government and about 40 senior armed forces officers, he warned that "those who fail to heed this call will be considered rebels".
"The country is in danger and needs all its children to defend it," he said.
Increasing tension
Mr Ravalomanana says he was cheated out of victory in last December's election.
Official results give neither him nor Mr Ratsiraka the 50% of the vote needed for outright victory and a run-off has been scheduled for later this month.
Mr Ravalomanana has refused to take part.
Apart from a few serious clashes which claimed the lives of several people, the showdown dividing the nation has been largely peaceful, though increasingly tense since Mr Ravalomanana declared himself president on 22 February.
Most members of the army have backed Mr Ravalomanana, government offices have been taken over and rivals installed as ministers while the population carry on as usual.