French left-wing firebrand Jean-Luc Melenchon has said he would run for the presidency again next year, a move that risks splitting the left-of-centre vote and hurting Socialist President Francois Hollande's chances of re-election.

Mr Hollande has tried to discourage candidates to his left, arguing this could prevent any left-of-centre politician reaching the second round run-off.

Mr Melenchon, 64, a former Socialist lawmaker who has become a staunch critic of Mr Hollande, could draw votes away from the president, who already faces an uphill battle for re-election due to public anger over high unemployment and other factors.

"I am presenting my candidacy for 2017," Mr Melenchon said on a video posted on his website. "This election can be a chance for the people. It is the opportunity to turn the page peacefully and democratically on an unfair and cruel system that our country and continent are sinking into."

Mr Melenchon gained 11.1 per cent of the votes in the first round of the last presidential election in 2012, helped by Communist backing.