By Richard Balmforth and Sarah White
PARIS (Reuters) - Centrist Emmanuel Macron clung on to his status as favourite to win France's presidential election in a four-way race that is too close to call, as the camp of far-right challenger Marine Le Pen ramped up its eurosceptic rhetoric in a row with Brussels.
A closely-watched Cevipof opinion poll published on Wednesday showed frontrunners Macron and Le Pen both losing some momentum ahead of Sunday's first round, and conservative Francois Fillon and far-left candidate Jean-Luc Melenchon still in contention for the second round run-off.
With millions of French voters still undecided or planning to abstain, the vote is the most unpredictable in France in decades and investors are nervous about potential last-minute surprises that could trigger market turmoil.
Le Pen and Melenchon, who both pitch themselves as defenders of French workers, say they could take France out of the European Union and the euro currency. Banks have requisitioned their staff to be at their desks through the night on Sunday to enable them to respond fast to the outcome.
Le Pen has pressed hard her anti-immigration, anti-globalisation message as she seeks to mobilise voters.
As she prepared for the last big rally of her campaign in the Mediterranean city of Marseille, her camp became embroiled in an angry Twitter exchange with the European Commission.
Reacting to Le Pen's refusal to appear on France's TF1 television channel on Tuesday unless the EU's yellow-starred blue flag was removed, the Commission tweeted: "Proud of our flag, a symbol of unity, solidarity and harmony between the people of Europe. Let's not hide it."
Le Pen's deputy Florian Philippot fired back: "You'll see, we'll soon be sticking your oligarchic rag in the cupboard."
The election race for a successor to the deeply unpopular Francois Hollande has become increasingly tense as the gap between the leading candidates shrinks.
SECURITY IN FOCUS
The Cevipof poll of 11,601 people showed first round support for Le Pen falling 2.5 percentage points since early April to 22.5 percent and backing for Macron down 2 points to 23 percent.
Melenchon, a firebrand left-winger who has surged in recent weeks, was on 19 percent, while Fillon, whose campaign has been hurt by a financial scandal, received 19.5 percent of support.
Macron would win a head-to-head contest against National Front chief Le Pen, the poll showed.
Another poll, a daily survey by Opinionway, gave similar projections to Cevipof for the top candidates and projected Macron beating Le Pen in the May 7 second round by 65 percent to 35.
Abstention, a key factor adding to uncertainty over the outcome of the first round, was seen at 28 percent, Cevipof's survey found - near a record level that helped Marine's father, Jean-Marie Le Pen, make it to the runoff in 2002.
Another poll, conducted by BVA, showed Macron taking 24 percent of the vote in the first round, one point more than Le Pen, with Fillon and Melenchon tied on 19 percent.
The BVA poll showed abstentions at between 20 and 24 percent.
Fillon, 63, an ex-prime minister whose campaign was derailed by an embezzlement inquiry targeting him, his wife and two of his children, got last-minute public endorsements from ex-president Nicolas Sarkozy and ex-prime minister Alain Juppe - two men he beat to win the presidential ticket of his party, The Republicans.
Fillon, who says he is victim of a "dirty tricks" campaign, said in comments reported by Le Parisien newspaper that he would work to ensure France's institutions better protected the confidentiality of sensitive information.
For large parts of the campaign, sleaze allegations have overshadowed hot button themes like unemployment and how to revive France's sluggish economy.
However, security and tackling the threat posed by Islamist militants has returned to the fore after the arrest of two men in Marseille on Tuesday suspected of plotting an imminent attack.
The Paris prosecutor said on Tuesday that a video linked to the two Frenchmen and intercepted in early April had featured a machine gun placed on a table as well as a newspaper which had one of the presidential candidates on the front page.
A source close to the investigation said on Wednesday that the candidate featured on the newspaper cutting was Fillon.
France's internal intelligence agency had warned the main candidates of a threat, campaign officials said.
(Additional reporting by Matthias Blamont, Sudip Kar-Gupta, Ingrid Melander, Emmanuel Jarry, Sophie Louet and Maya Nikolaeva; Editing by Richard Lough and Adrian Croft)