PARIS, June 17 (Reuters) - Campaigning kicked off in France on Monday for a 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 triggered a sell-off 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 three weeks to prepare for the ballot, could back-fire, a poll by Ifop for the Journal du Dimanche showed.

It saw the eurosceptic, anti-immigration RN getting 35% of the votes in the first round on June 30, with 26% for the newly agreed, fragile alliance of leftwing parties, and just 19% for Macron. The second round will take place on July 7.

Official campaigning began on Monday, after a week in which parties scrambled to field candidates and strike alliances.

Macron's allies repeated warnings that a victory for RN, or for the left, could create a financial crisis. A victory for either would be catastrophic for France, its economy and jobs, Prime Minister Gabriel Attal told RTL radio.

Macron gathered key ministers and aides on Sunday evening to discuss the election, a source who took part in the meeting said, adding that they decided not to field any candidates in about 60 constituencies - out of 577 - where they considered that another mainstream candidate was in a better position to win.

But a decades-old consensus among mainstream parties to join forces to keep the far right out of power appears to have disintegrated and some in Macron's camp expressed doubts publicly about the snap election.

"This (dissolution of parliament) is the decision of the president, it's his prerogative," Finance Minister Bruno Le Maire told France Inter radio on Sunday.

"What I observe is that it has created in our country, among the French people, everywhere, worries, incomprehension, sometimes anger. That's what I see among our voters."

The RN, which has already said it would slash VAT on energy and lower the retirement age, is set to detail its economic programme in the coming days. (Reporting by Ingrid Melander, Elizabeth Pineau, Sudip Kar-Gupta; editing by Christina Fincher)