With France's presidential race tightening ahead of Sunday's first-round vote, favorites President Emmanuel Macron and far-right challenger Marine Le Pen battled over pension reform on Monday.

Opinion polls have long predicted Macron will win a second term but Le Pen has tightened the gap, with polls showing the 44-year old president with only a six-point advantage in a likely run-off on April 24.

Le Pen has benefited from a campaign focused on purchasing power on which she doubled down on Monday.

"Do you realize what retirement at 65 is? It's simply completely unfair," she told BFM TV, lambasting Macron's plan to increase the legal age at which one gets a full pension from 62 to 65.

Le Pen wants to keep the 62-year-old threshold and bring it down to 60 for those who started working before age 20. Pushing back the retirement age would hurt workers, she said, arguing that many would not manage to find a job at that age and would see their pension hit as a consequence

Macron, asked about criticism of his pension reform plans, told France Inter radio, "Those who tell you we can keep (the pension system) as it is now are lying to you."

Raising the retirement age – with exceptions for those who have tough jobs or worked longer than others – was needed to make the system viable and increase low pensions, he said.

Macron, when he belatedly entered the election campaign last month, said he would increase the retirement age, cut taxes and further loosen labor market rules, seeking a mandate to press on with pro-business reforms.

Stressing his pro-business credentials was not without risk as households feel the squeeze from rising prices and could put off a number of left-wing voters from backing him against Le Pen in a likely run-off on April 24.

On Saturday, in his only campaign rally before the first round, Macron tried to convince voters of the risk of a Brexit-style election upset that could see Le Pen take the far-right to power in France.

"Look at what happened with Brexit, and so many other elections: what looked improbable actually happened," he said. "Nothing is impossible."

Even if Macron does win a second mandate, as polls still expect, the issue of pension reform, which dogged his first term, could be a problem, considering how widespread the opposition is.

One first, major challenge would be for his center-right La Republique en Marche (LaRem) party, which has failed in all recent local elections, to win a parliamentary election in June.