By Richard Balmforth
PARIS (Reuters) - French centrist Emmanuel Macron and far-right leader Marine Le Pen clung on as frontrunners in France's tight presidential race on Tuesday, but the unpredictable outcome is pushing some pollsters to calculate the most extreme runoff scenarios.
In a new twist in the two-round election, Jean-Luc Melenchon, a far-left veteran who for most of the campaign has been dismissed as a distant no-hoper, has surged into the top four and lies just a few percentage points behind the leaders.
Though some commentators see Melenchon's challenge as a blip that may fade, his rise has injected further uncertainty into the outcome of the race for the Elysee, in which Macron has largely been seen as the favorite.
Some investors are even weighing up the possibility of Melenchon making it into the second round against Le Pen, a clash between two far-left and far-right arch-rivals that would stand French politics on its head.
The turbulent presidential campaign has grown increasingly bitter in recent weeks as candidates eye the finishing line.
With the first round of voting due on April 23, when a field of 11 candidates will be whittled down to two, Macron and conservative rival Francois Fillon, who are each fighting for center-right votes, sniped at each other's programs.
Fillon, a former prime minister who has been holding onto the third place in polls despite a scandal over payments of public funds to his family that has hurt his campaign, called Macron a liar.
Macron responded on Sud Radio: "Mr Fillon is a man of little worth."
Polls showed Macron and Le Pen, leader of the anti-immigrant and anti-EU National Front, still several percentage points ahead of Fillon and Melenchon in the first round - something which would send them through to a face-off with each other on May 7.
Pollsters Elabe, in a survey carried out for media groups L'Express and BFMTV, saw them both on 23 percent, half a point down from a similar poll last week. The Elabe poll had Fillon on 19 percent, with Melenchon on 17 percent.
Elabe projected that Macron, a former banker and economy minister in a Socialist government, would go on to beat Le Pen comfortably in the May 7 runoff. Other polls have shown a similar picture.
An estimated one in three voters, however, remain undecided over who they will plumb for in the first round. Commentators therefore caution that the picture could easily change with the fortunes of candidates being affected by a sudden misstep on the stump or an ill-chosen word.
Macron, who says he wants to transcend the left and the right in politics, leads a fledgling movement called En Marche! (Onwards!) and has never fought for, or held, elected office. But many senior Socialists have defected to his cause.
Fillon, on the other hand, has the weight of the party machine of The Republicans behind him to help counter the financial scandal in the final days of campaigning.
Speaking in Marseilles on Tuesday night, Fillon said he was sure of getting through the first round and urged his supporters not to heed the opinion surveys.
"They are trying to dictate your choice to you with these polls. They are suggesting to you that you don't need to vote because everything is already decided," he said.
Melenchon's verve as a political showman, who uses strong rhetorical skills to call for a 100 percent tax on the rich and an exit from NATO, has made him a huge success on talk shows and televised debates.
Though he has picked up most of his votes from the official Socialist candidate Benoit Hamon, whose campaign is in nose-dive, Melenchon also targets centre-left voters who are tilting toward Macron.
Melenchon's advance in the polls has spooked financial markets and prompted a warning on Tuesday by the head of business lobby group Medef Pierre Gattaz.
Calling both Melenchon and Le Pen's programs "an absolute catastrophe" for France, Gattaz warned on Europe 1 radio against a possible Melenchon-Le Pen second round.
Unusually, the Elabe poll of around 1,000 voters on Tuesday tested a variety of hypothetical second-round matchups.
It found that Macron would beat Le Pen, Fillon or Melenchon. Melenchon would beat Le Pen or Fillon, but lose to Macron. Fillon would beat Le Pen but lose to either of the other leading candidates.
Melenchon is on the opposite end of the political spectrum to Le Pen, in particular on immigration. But they both distrust the European Union, want to renegotiate France's role in it and to hold a referendum on EU membership. Their social policies, including on workers' protection, are also close.
Le Pen vowed on Tuesday to take 10 concrete measures in the first two months of winning election.
These included suspending France's participation in the EU's Schengen passport-free travel zone and re-establishing national border checks, and expelling all foreigners being monitored by intelligence services as possible threats to national security.
(Reporting by Sarah White, Sophie Louet and Helene Dauschy, Sudip Kar-Gupta, Jean-Francois Rosnoblet; Writing by Richard Balmforth; Editing by Mark Heinrich)