* Voting starts at 0600 GMT, first results estimates at 1800 GMT

* Macron is ahead in opinion polls

* Choice is between pro-European centrist, far-right eurosceptic

By Ingrid Melander, Elizabeth Pineau and Layli Foroudi

PARIS, April 24 (Reuters) - The French vote on Sunday in an election that will decide whether pro-European Union, centrist President Emmanuel Macron keeps his job or is unseated by far-right eurosceptic Marine Le Pen in what would amount to a political earthquake.

Opinion polls in recent days gave Macron a solid and slightly growing lead as analysts said Le Pen - despite her efforts to soften her image and tone down some of her National Rally party's policies - remained unpalatable for many.

But a surprise Le Pen victory cannot entirely be ruled out, given the high numbers of voters who were undecided or not sure if they would vote at all in the runoff presidential vote.

With polls showing neither candidate able to count on enough committed supporters, much will depend on a cohort of voters who are weighing up anxiety about the implications of a far-right presidency against anger at Macron's record since his 2017 election.

If Le Pen does win, it would likely carry the same sense of stunning political upheaval as the British vote to leave the European Union or the U.S. election of Donald Trump in 2016.

Polls open at 8 a.m. (0600 GMT) and close at 8 p.m. (1800 GMT). Initial projections by pollsters are expected as soon as polls close.

"Each of them has a huge weakness," Bernard Sananes of pollster Elabe said. "Emmanuel Macron is considered arrogant by more than one in two voters and Marine Le Pen remains scary for half of them."

Macron, 44 and the winner in the same matchup five years ago, has warned of "civil war" if Le Pen - whose policies include a ban on wearing Muslim headscarves in public - is elected, calling on democrats of all stripes to back him against the far-right.

Le Pen, 53, has focused her campaign on the rising cost of living in the world's seventh largest economy, which many French say has worsened with the surge in global energy prices. She has also zeroed in on Macron's abrasive leadership style, which she says shows an elitist contempt for ordinary people.

"The question on Sunday is simple: Macron or France," she told a rally in the northern French town of Arras on Thursday.

Le Pen's message has resonated with many voters.

"She is close to the people. She can really give purchasing power to the people, make the people smile, give the people oxygen," prison guard Erika Herbin, 43, said after the rally.