"For my part, I will vote for Emmanuel Macron," Hollande announced in a televised address on Monday afternoon.
"The presence of the far right [in the second round of voting] once again poses a risk for our country," he went on, warning that Le Pen's plans to quit the eurozone and introduce protectionist policies would destroy jobs, reduce living standards and push up prices.
The National Front would "deeply divide France" in the face of the "terrorist threat", the president said.
European Union Finance Commissioner Pierre Moscovici, a former minister in Hollande's government, also pledged to vote Macron, calling Le Pen's 7.6 million score was "frightening".
"The European Commission is ready to work with the new French president on how we can re-launch Europe," he said.
Mayor threatens to resign
A less well-known French Socialist also reacted angrily to Le Pen's showing.
Daniel Delomez, who is mayor of Annezin in north-west France, declared that he might resign because "I don't want to spend my life working for idiots" after learning that 38.09 percent of the villagers had voted for the National Front candidate.
Delomez voted for hard-left candidate Jean-Luc Mélenchon, who won 19.25 percent in Annezin, and said he would vote Macron in the second "without hesitation".
After a local paper report of his comment caused a storm on social media, he said the phrase had been an overreaction but that he was still thinking of resigning.
Russia denies backing Le Pen
Going further afield, Russia on Monday denied having any preference in the election.
"We respect the French people's choice," Kremlin spokesperson Dmitri Peskov told journalists on Monday.
It was untrue that Russia supports any particular candidate, he said, although Le Pen met President Vladimir Putin in March and favours closer relations with Moscow.
Peskov again denied Russian involvement in hacker attacks on Macron's website during the campaign.