The pair will face each other in a run-off vote on 7 May after taking the top places in Sunday's first round, with Mr Macron the current favourite to win.
Marine Le Pen, 48
Mr Hollande said a far-right victory would endanger the country.
He said: "What is at stake is France's make-up, its unity, its membership of Europe and its place in the world."
His brief TV address on Monday reflected a move by much of France's mainstream to line up behind Mr Macron to try to stave off Ms Le Pen.
Earlier, defeated candidates, the Republicans' François Fillon and Socialist Benoît Hamon, both urged supporters to vote for Mr Macron.
Ms Le Pen quickly renewed her attacks on Mr Macron on Monday, calling him a "weakling" for his anti-terrorism policies.
Emmanuel Macron, 39
The victory of Ms Le Pen and Mr Macron meant that, for the first time in six decades, neither of France's main left-wing or right-wing parties had a candidate remaining in the election.
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