In a televised statement, the Socialist leader said France risked "becoming isolated and breaking away from the European Union" if the far-right, anti-EU Le Pen were elected.
Hollande said French purchasing power would be hit, "thousands" of jobs would be lost and prices would soar if Le Pen won and then implemented her pledge to take France out of the eurozone and hike tariffs on imports.
A far-right victory would also "deeply divide France" at a time when it needed to show "solidarity and cohesion" in the face of terrorism, he said.
"Faced with such a risk it is impossible to remain silent or indifferent," he said, adding: "For my part, I will vote for Emmanuel Macron."
Macron was Hollande's economy minister from 2014 to 2016 when he quit to form his own centrist movement En Marche (On The Move).
On Sunday, Macron topped the first round of voting for president, winning 23.75 percent of the vote to Le Pen's 21.53 percent.
Nine other candidates were eliminated.
Macron and Le Pen will face off in a second round of voting on May 7.
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