PARIS, April 10 (Reuters) - Voting was under way in France on Sunday in the first round of a presidential election, with far-right candidate Marine Le Pen posing an unexpected threat to President Emmanuel Macron's re-election hopes.

Polling stations opened at 8 a.m. (0600 GMT) and will close at 1800 GMT, when the first exit polls will be published. Such polls are usually very reliable in France.

Until just weeks ago, opinion polls pointed to an easy win for the pro-European Union, centrist Macron, who was boosted by his active diplomacy over Ukraine, a strong economic recovery and the weakness of a fragmented opposition.

But his late entry into the campaign, with only one major rally that even his supporters found underwhelming and his focus on an unpopular plan to increase the retirement age, have dented the president's ratings, along with a steep rise in inflation.

In contrast, the anti-immigration, eurosceptic far-right Le Pen has toured France confidently, all smiles, her supporters chanting "We will win! We will win!".

She has been boosted by a months-long focus on cost of living issues and a big drop in support for her rival on the far-right, Eric Zemmour.

Opinion polls still see Macron leading the first round and winning a runoff against Le Pen on April 24, but several surveys now say this is within the margin of error.