A firebrand leftist has emerged as a powerful force in France's presidential election, gaining popularity thanks to a no-holds-barred style that has forced the mainstream candidates to pay heed to his capitalist-bashing agenda.

Jean-Luc Melenchon, a one-time Trotskyist and former teacher who co-heads a group of far-left parties, has become a favourite on talk shows where he rails against greedy banks, overpaid chief executives and a European Union he says is squeezing its m0embers dry.

After an explosive TV appearance with far-right chief Marine Le Pen, in which she refused to debate with him and he accused her of distilling hatred, Melenchon's support surpassed the symbolic 10 percent mark for the first of two election rounds, according to a survey last week by pollster CSA.

That score gives Melenchon little chance of reaching a May 6 run-off against President Nicolas Sarkozy or his Socialist rival Francois Hollande but it puts him close to second-tier centrist and far-right candidates polling behind them.

As polls point to growing disappointment with the presidential campaign, which most voters say puts too much focus on petty exchanges between Sarkozy and Hollande, Melenchon is gaining ground with a scowling yet eloquent appeal to the electorate.

The difference in numbers at our meetings is palpable, said Martine Billard, co-head of the Left Front with Melenchon. In our campaign stops, in the street, everybody wants to stop and talk with Jean-Luc ... In the past few weeks we really have the feeling of a strong dynamic.

Melenchon is hotly tipped to take a ministerial post in the case of a Socialist victory, provided he backs Hollande for the May 6 second round.

Scope for a breakthrough by a fringe candidate, including by Le Pen, credited with around 15 percent support, has narrowed in the past few weeks as the election race takes shape as a binary clash of personalities between the two front-runners.

On Sunday, Sarkozy recast himself as France's saviour from low-cost competition and high immigration and threatened to disregard European limitations on protectionism as he sought to give his campaign a second wind.

And yet Melenchon, a former minister who quit the Socialists in 2008 to found his Party of the Left, has pulled the campaign in his direction, forcing both leaders to toughen their stance on taxes, wealth inequality and executive pay in a campaign marked by acute economic anxiety.

His voice captures anti-liberal feelings which are close to where the country stands ideologically, said Stephane Rozes, head of the Cap political consultancy. Add to that his blue-collar charisma and a bit of old-school anarcho-syndicalism and you have a real political force.

I TAKE IT ALL!

With a tone that is part working class toughness, part literary refinement, Melenchon has resurrected a style from the days when French Communists routinely pulled in between 10 and 20 percent of votes in presidential elections.

On his blog, which features posts titled The Rich and the Powerful, The Insurrection Starts Now and It's About to Get Bloody, he writes about the difficulty of holding back from quoting Marxist thinkers in his speeches.

Melenchon directs his most virulent jibes at Le Pen, whom he has called a bat and a dark presence. But Hollande has also felt the sting, accused of being a pedal boat captain by Melenchon in a Socialist primary vote in October.

When Hollande told British daily The Guardian that there were no communists left in France, Melenchon accused him of having an unbearably haughty attitude and obtained an apology from the would-be president.

Asked if Melenchon's tone was a calculated bid for attention, the Left Front's Billard said: He is not making any apologies... such urgent times demand that sort of language.

Melenchon, whom Socialists have accused of trying to sabotage Hollande's bid, has pledged to back the frontrunner after the first-round vote. But he has used his clout to pressure Hollande into adopting a clearer left-wing line.

Hollande garnered only mild praise from Melenchon with plans for a 75 percent tax on annual incomes above a million euros, which the latter said did not go far enough. Melenchon's platform calls for a cap on annual revenue at 340,000 euros (285,000 pounds), with any earnings above that to be confiscated by the state.

The idea harkens back to a 1981 campaign pledge by Georges Marchais, a legendary Communist boss and precursor to Melenchon, who said at the time: Above 40,000 francs, the tax is 100 percent -- I take it all!

Sarkozy is also paying attention. While rejecting Hollande's supertax, he has made the fight against outsourcing a key plank of his campaign and started to pepper his speeches with references to the mental anguish experienced by the unemployed.

He has also said that if re-elected he would change the law to give workers seat on company boards so they could oversee executive pay, as well as the end of hefty severance packages.

This is a bad joke... obviously it will change nothing because the shareholder's decision will take precedence, Melenchon wrote on his blog. In the face of such greed, laws and taxes are the only effective response.

(Editing by Daniel Flynn and Toby Chopra)