Campaigning kicked off on Monday for France's snap parliamentary election which opinion polls suggest the far-right National Rally will win, with President Emmanuel Macron's centrist alliance coming third, behind a leftwing ticket.

Political uncertainty has triggered heavy selling of French bonds and stocks after Macron unexpectedly called the election, following a trouncing of his ruling centrist party by Marine Le Pen's National Rally (RN) in European Parliament elections.

Macron's gamble, which included catching other parties off-guard with just a few weeks to prepare for the ballot, could back-fire, a poll by Ifop for the Journal du Dimanche showed.

The poll suggests the eurosceptic, anti-immigration RN will get 35% of the votes in the first round on June 30, with 26% for a fragile alliance of leftwing parties, and just 19% for Macron. 

The second round will take place on July 7.

"We're going into uncharted territory, and in my opinion, we are going to move towards an ungovernable Assembly," 60-year-old voter Maxime Chetrit said.

Marie Balta, a retiree from Nimes, in southern France, shared this concern, but said the election could give parliament more power over what the president and government do.

"It's going to be very difficult, to have a tripartite Assembly with two strong blocks and a much smaller middle, but it's perhaps a chance to return to more democracy," she said.