PARIS (Reuters) - France's Francois Fillon will convincingly beat National Front leader Marine Le Pen in a presidential election runoff next May, taking 66 percent of the vote, a new opinion poll showed on Wednesday.

The Elabe poll showed free-marketeer conservative Fillon, who secured his Les Republicains party nomination on Sunday, remains firmly in first position among several declared or potential candidates, including Le Pen and socialist incumbent Francois Hollande.

Both Fillon and Le Pen, head of the anti-immigrant, anti-EU, National Front, are expected to make it to a head-to-head runoff on May 7 after a first round of voting in April which will winnow out the field.

In that runoff, Fillon would likely secure 66 percent of the votes versus 34 percent for Le Pen, according to the poll, which was conducted by Internet on Sept. 28 and 29 and involved a representative sample of 941 voters.

The poll suggested the third-placed candidate at this stage would be Emmanuel Macron, a former investment banker and economy minister in Hollande's government who has decided to run as an independent candidate.

The poll showed far-left candidate Jean-Luc Melenchon in fourth place ahead of Hollande, who is deeply unpopular and has yet to say whether or not he will seek re-election.

(Reporting By Brian Love; Editing by Richard Balmforth)