Who is he?
Jean-Luc Mélenchon heads up the hard-left “France Insoumise” (Unsubmissive France) coalition, backed by the French Communist Party and has become the surprise challenger in France's presidential race.
The 65-year-old former Socialist minister is also the Member of the European Parliament (MEP) for South West France.
Born in the Moroccan port city of Tangiers, Mr Mélenchon attributes his fiesty character to his Mediterranean roots.
He studied philosophy and was a Trotskyist student activist before joining the Socialist party at age 25. He became the youngest member of the French Senate in 1986. Later he served as vocational education minister under Socialist premier Lionel Jospin from 2000 to 2002.
But in 2008, he fell out with then party leader Francois Hollande and quit the Socialists, saying "our country needs another voice on the left".
Mr Mélenchon ran in the 2012 Presidential election. He came fourth in the polls, and was hailed as a powerful orator and performed strongly in debates with the other candidates.
An admirer of late Venezuelan president Hugo Chavez and Chinese chairman Mao Zedong, Mr Mélenchon sprinkles his tirades against the liberal establishment with odes to the arts and philosophy.
An innovative campaigner, he has also created a buzz by appearing at rallies as a hologram and boasts more than a million followers on Twitter and his own YouTube channel - a way to circumvent the traditional media, which he accuses of bias.
What does he want?
Among Mr Mélenchon's most eye-catching policies are a 100-billion-euro economic stimulus plan funded by government borrowing, corporate nationalization in sectors such as the motorway network, devaluation of the euro currency, withdrawal from NATO and a possible "Frexit" from the European Union.
He also pledges a rewrite of the Constitution to end what he calls a "presidential monarchy" - shorthand for a radical change that would boost parliamentary power and voter control over a head of state who currently enjoys more power in his country than presidents in many other Western democracies, including the United States.
Other policies include environmental protection and increased labour rights. In the past he has proposed a 100 per cent income tax on French residents who earn over €360,000 a year.
Mr Mélenchon has also called for the debt of troubled eurozone states to be written off to allow massive new investment to spur growth.
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Mr Mélenchon is accused by detractors of cosying up to Vladimir Putin and other autocratic leaders in South America while blaming the West for most of the world's conflicts.
"He explains to us that democracy is Cuba, Venezuela, Russia," said Yannick Jadot, an ecologist who supports Socialist candidate Benoit Hamon.
"Putin is the one who validates chemical weapons massacres in Syria and an absolutely totalitarian regime in Chechnya that is the process of massacring homosexuals," he said.
Critics say his anti-German stance smacks of nationalism and is unlikely to curry favour with Chancellor Angela Merkel.
In his 2015 book Bismarck’s Herring (The German Poison), Mr Mélenchon wrote that “Germany is again a danger”, its “imperialism” is “returning”, and the EU is its “new empire”. "A monster is born across the Rhine," he exclaimed.
Mr Mélenchon has also described Germans as “grumbling Teutons” who seek to “deport” their old people to eastern Europe or Thailand.
And he’s written that German “expansionism” was at work in the country’s 1990 reunification – an “annexation” of East Germany, in his words.
Meanwhile, Pierre Gattaz, head of the French employers' federation MEDEF, warned his election would spell "absolute catastrophe", as would far-Right leader Marine Le Pen. "Mélenchon is the Venezuela scenario, Le Pen is the Argentine scenario," he said.
Can he win?
Opinion polls show the top four candidates - centrist Emmanuel Macron, far-right leader Marine Le Pen, conservative Francois Fillon and Mr Mélenchon - within touching distance of each other before the April 23 first round after a surge by Mr Mélenchon in the last few weeks.
The polls tip Macron to beat National Front leader Le Pen in a runoff between the two leading candidates on May 7.
Whatever happens, it will be close.