Speaking from the Elysee palace, Mr Hollande said Ms Le Pen's platform of pulling out of the euro would devastate the country's economy and threaten French liberty.
He said the far-right would "deeply divide France" at a time when the terror threat requires "solidarity" and "cohesion".
Mr Macron and Ms Le Pen, leader of the National Front, will go head-to-head on 7 May after taking the top two places in the first round.
Opinion polls indicate Mr Macron, who has never held elected office, will take at least 61 per cent of the vote against Ms Le Pen after two defeated rivals pledged to back him to thwart her eurosceptic, anti-immigrant platform.
Mr Hollande, a Socialist nearing the end of five years of unpopular rule, threw his weight behind his former economy minister in a televised address, saying Ms Le Pen's policies were divisive and stigmatised sections of the population.
"The presence of the far-right in the second round is a risk for 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."
Global markets reacted with relief to Sunday's vote, which broke the dominance of established parties of the centre-left and centre-right but still left the most market-friendly and internationally-minded of the remaining contenders in pole position to become France's next leader.
Opening the battle for second-round votes, Ms Le Pen highlighted the continuing threat of Islamist militancy, which has claimed more than 230 lives in France since 2015, saying Mr Macron was "to say the least, weak" on the issue.
Ms Le Pen has promised to suspend the EU's open-border agreement on France's frontiers and expel foreigners who are on the watch lists of intelligence services.
Mr Macron's internal security programme calls for 10,000 more police officers, and 15,000 new prison places, and he has recruited a number of security experts to his entourage.
However, opinion polls over the course of the campaign have consistently found voters to be more concerned about the economy and the trustworthiness of politicians.
Whichever candidate wins on 7 May will need to try to build a majority six weeks later in a parliament where the National Front currently has only two seats and Mr Macron's year-old En Marche! (Onwards!) movement has none.
Mr Macron has already enlisted some 50 sitting Socialist MPs to his cause, as well as a number of centrist party grandees.
Additional reporting by agencies