Macron is the clear favourite to become France's youngest-ever president after topping Sunday's first round of voting with 24.01 percent of votes, ahead of National Front (FN) leader Le Pen on 21.3 percent, according to final results.
The vote revealed a country deeply divided, with 39-year-old Macron, an advocate of open borders and free trade, leading the vote in cities and Le Pen topping the polls in rural areas that feel left behind by globalisation.
In a solemn address to voters, Hollande warned of the "risk for our country" of a far-right victory and said he himself would vote for Macron, who served as his economy minister for two years.
The Socialist president joined a long line of politicians urging voters to back Macron in order to thwart the anti-immigration, anti-EU Le Pen.
WATCH: Outsiders race in French presidential election
Le Pen seized on the flurry of endorsements for Macron from the ruling Socialists and main opposition Republicans -- both of which crashed out in the first round -- as proof he was the choice of a discredited old guard.
Visiting a market in the northern town of Rouvroy on Monday, 48-year-old Le Pen lashed out at the "rotten old republican front" -- the ad-hoc anti-FN coalition formed by mainstream parties whenever the party is at the gates of power.
"I've come here to start the second round campaign in the only way I know -- on the ground with the French people," she said.
She later told France 2 television: "Nothing in either Mr Macron's policies or his behaviour suggests the slightest proof of love for France," and insisted, "We are going to win."
Le Pen, who is also FN leader, also said she was setting party affairs to one side in order to concentrate on the campaign.
"I will feel freer, I will be above partisan considerations, it's an important act," Le Pen said.
Macron received several congratulatory calls on Monday from European leaders relieved to find him in pole position.
Thanking Hollande in a tweet for his support, the former investment banker called on the French to "remain true to France's values" in the May 7 runoff.
He and Le Pen will take part in a TV debate on May 3.
According to two opinions that were carried out on Sunday night, Macron would beat Le Pen by 62-64 percent over 36-38 percent.
I want to become your president: Macron
'Healing wounds'
Sunday's results were a stunning blow to France's traditional political class, with voters fleeing the ruling Socialists and conservative Republicans who have governed for the past half-century.
Both Macron and Le Pen campaigned as rebels who transcended the left-right divide.
Republicans candidate Francois Fillon was seen as the favourite until January when his campaign was torpedoed by allegations that he gave his British-born wife and two of his children fictitious jobs as parliamentary assistants.
In the end, Fillon trailed in third with 20.01 percent, ahead of radical left candidate Jean-Luc Melenchon on 19.58 percent.
On Monday, Fillon told the party's top brass he lacked "the legitimacy" to lead them into parliamentary elections in June.
"I need to rethink my life and heal my family's wounds," he told them, according to a statement.
Fillon and fifth-placed Socialist candidate Benoit Hamon have both rallied behind Macron.
But Melenchon has pointedly avoided backing the centrist. The leaders of the Republicans squabbled Monday over whether to endorse Macron openly or simply urge supporters not to vote for Le Pen.
'Patriots v nationalists'
Addressing thousands of flag-waving supporters in Paris on Sunday evening, Macron said he aimed to unite "patriots" against "the threat of nationalists".
Le Pen, who has been hoping to emulate Donald Trump's victory in the US, said the French faced a choice between "runaway globalisation" and a protectionist France.
Her plans to restore France's borders with its European neighbours, pull out of the eurozone and hold a referendum on leaving the EU had sown fear of another devastating blow to the bloc after Britain's vote to leave.
The euro and European stocks rose Monday, buoyed by expectations that Macron is the hot favourite for next president.
Le Pen gained over 1.2 million new voters compared with her last presidential bid in 2012, securing 7.7 million ballots, a result she hailed as "historic".
But her share of the vote was far below a March poll high of 27 percent.
She follows in the footsteps of her father Jean-Marie, who made it through to the 2002 presidential run-off in what was a political earthquake for France.
Le Pen senior went on to suffer a stinging defeat when mainstream parties closed ranks to keep him out.
Sunday's first round capped a rollercoaster campaign in a demoralised France, which has been rocked by a series of terror attacks since 2015 and is struggling to shake off a deep economic malaise.
