Support for far-left candidate Melenchon and his far-left France insoumise alliance - which can be translated as "Unbowed France" - was at 19 percent, according to the Cevipof poll.
French presidential hopeful Emmanuel Macron has had a phone call with former U.S. President Barack Obama.
Nevertheless, neither of the Eurosceptic two are frontrunners to win the eventual presidency.
(Christian Hartmann/Pool photo via AP).
The first round of the French election will be held this Sunday April 23.
The leftist leader recently spoke in Marseille, where he described his candidacy as a third option for voters who are dissatisfied with extreme right-wing politics and supporters of free market economics.
Four days before the first round of the French presidential election, a new poll has shown front-runners Emmanuel Macron and Marine Le Pen losing some of their momentum.
Candidates on the right have been especially vocal, seeking to appeal to voters traumatized by Islamic State group-inspired attacks that have killed at least 235 people in France since January 2015, by far the largest casualty rate of any Western country. Tight security was the order of the day for Le Pen and other top candidates.
What is at stake in this election is the continuity of France as a free nation, our existence as a people, she said in February 2017.
Speaking on BFM television yesterday, Le Pen emphasised how she would pull France out of the European Union, slash immigration, make it harder to get French nationality and crack down on suspected Islamists.
"If you look at the two markets, or the most recent markets of the American election obviously and Donald Trump, you look at Brexit, we didn't predict them and they happened".
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Crowds danced on a Paris plaza as Socialist presidential candidate Benoit Hamon held what is seen as a last-chance rally and concert.
While analysts have long held that the presidential election would require a second round of voting, predicting which two candidates will emerge for the winner-take-all ballot is becoming increasingly hard.
Hamon won the Socialist primary but the party is deeply divided, and Socialist President Francois Hollande is so unpopular that he's not seeking a second term. He took a swipe at Le Pen for systematically linking immigration and terrorism, and said Fillon's promise to recruit more police was not credible as he had destroyed police and army jobs when he was President Nicolas Sarkozy's prime minister.
Ahead of his final event in the western city of Nantes, Macron met with the country's RAID intervention force accompanied by Defense Minister Jean-Yves Le Drian.
"This vast army in the shadows who want us to live in fear.is not a fatality", she told the cheering crowd.
Le Pen has proposed expelling any foreigner convicted of a crime or suspected of being radicalised.
(AP Photo/Laurent Cipriani). A hologram of French Left party leader and candidate for the 2017 French presidential election Jean-Luc Melenchon speaks to supporters who gathered in Grenoble, southeastern France, Tuesday, April 18, 2017.
Fillon - an early favourite before his campaign hit the rocks following nepotism allegations that he has denied - scored 20 percent, meaning he was now gaining on Le Pen.
After his meeting with Anouar Kbibech, head of the French Muslim federation CFCM, Macron issued a statement insisting on the importance of respecting France's secular traditions but saying those shouldn't be used to target Muslims.
Le Pen has said she would extend the law banning "ostensible" religious signs to the streets of France.