France on Saturday prepared for its most consequential legislative election of recent times, with residents of overseas territories opening voting for a poll expected to give the far right its biggest ever presence in parliament.

A traditional final day pause was observed on Saturday ahead of Sunday's second round runoff after a frenetic campaign that saw tensions rise across the country and dozens of attacks on candidates.

Underlining France's global footprint that spans the oceans of the world, the first French region to vote was Saint Pierre and Miquelon, a small French archipelago off the coast of Canada where citizens began voting from 1000 GMT.

They were to be followed on Saturday by residents of French Caribbean territories including Martinique and Guadeloupe as well as Guiana in South America.

French Pacific territories come next before people in mainland France cast their ballots from 0600 GMT on Sunday.

Polls will close by 1800 GMT on Sunday when projections of seat numbers -- regarded in France as a firm indicator of the final outcome -- are published.

Final opinion polls on Friday suggested that far-right leader Marine Le Pen's National Rally (RN) would fall short of winning an outright majority in the National Assembly.

But President Emmanuel Macron's gamble in calling snap elections is expected to end with his centrist alliance having half the number of deputies it had before.

He now faces the final three years of his presidency with no clear ruling majority, and Prime Minister Gabriel Attal perhaps trying to hold together a caretaker government.

"Today the danger is a majority dominated by the extreme right and that would be catastrophic," Attal said in a final pre-election interview with French television on Friday.

Le Pen insists that she is still on course for victory and an absolute majority that would force Macron to appoint her 28-year-old lieutenant Jordan Bardella as prime minister.

Attal vowed to stay on "as long as necessary" in a caretaker role, while Macron's office is studying options to maintain some form of government.

Macron is to remain in office until presidential and legislative elections in April 2027, but he must now face the possibility of sharing power with political foes. (AFP)