Anti-capitalist firebrand whose ideas include 100% fat cat tax on earnings above £300,000 wins attention and support in the polls

Jean-Luc Mélenchon moves from left to centre stage in battle to be president

Jean-Luc Mélenchon, the hard left, anti-capitalist firebrand who is rising in the presidential election polls, is all over the French papers – billed as the great surprise, main event and key revelation of the campaign.

With crowds spilling into the street at his packed rally in Lille this week, and tens of thousands recently flocking to the Bastille to hear him call for a "civic insurrection", Mélenchon has been credited with 14% in the polls by BVA.

His numbers have catapulted him into the realms of becoming a possible "third man" in the first round vote on 22 April.

Mélenchon's initial aim is to overtake and annihilate his arch-nemesis, the National Front's Marine Le Pen, in the race for the working class and protest vote, as he explained on France Info radio on Thursday.

Mélenchon is already famous for his scathing attacks on opponents, and his public savagery of what he terms the "half-demented" Le Pen has become the stuff of campaign legend.

But commentators are now wondering whether François Hollande, the Socialist frontrunner, should be worried that Mélenchon's rise will eat into his vote.

Most of Mélenchon's voters are predicted to transfer to Hollande in the second round, as the broad French left is keen to do anything to stop Nicolas Sarkozy.

Some say the current Mélenchon-mania is good for the left, boosting its overall score. Others, who want Hollande to be the clear winner in the first round before the final runoff, say Mélenchon's rise should be contained.

So far, Hollande's strategy has been to carry on much as usual, pressing home his ideas such as a 75% tax on income over the €1m (£836,000) mark, and arguing that a huge, strategic rallying Socialist vote is needed in the first round in order to beat Sarkozy in the 6 May runoff.

Arnaud Montebourg, on the left of Hollande's Socialist team, on Thursday called on Mélenchon to save any aggressive attacks for Sarkozy, not Hollande, warning that the right loves to delight in any cracks on the left.

Mélenchon, a one-time Trotskyist and former teacher, spent 30 years in the Socialist party, where he served as a minister and senator.

He called for a no vote on the European constitution in 2005 before leaving to co-found his own Parti de Gauche. He is now running for a leftist alliance, Front de Gauche, which includes the once powerful Communist party.

His ideas include capping maximum fat cat salaries at €360,000 (£300,000), after which income tax would be set at 100%.

Historically, the French "left of the left" usually takes 10%-12% of the first round presidential vote, split between various candidates, including the Trotskyists.

This time Mélenchon has siphoned voters from the smaller far-left candidates and focused the radical spotlight on himself.

People question whether he wants a government role, but he swears he will not sit in a Socialist party government. He also says there will be no negotiations.