Far-right French presidential candidate Marine Le Pen took a fishing-boat ride on Thursday as two polls suggested the underdog had made a more impressive start to the last lap of campaigning than the favourite, centrist Emmanuel Macron.

Since last Sunday's opening ballot sent them into a two-way runoff on May 7, the battle has intensified, notably on the public relations front, between two candidates who both say their adversary will ruin the country.

A daily Opinionway poll showed Macron still clear favourite, but his predicted score, which has almost always been 60 percent or higher over the past few months, dipped to 59 percent for the first time since mid-March.

A separate Elabe poll also signalled a potential danger for the favourite: it said one out of two people surveyed considered Le Pen's last-leg campaign had begun well, while for Macron that positive view of latest developments was a slimmer 43 percent.

The progression of Macron and Le Pen to the second round on April 23 sent the euro sharply higher and lifted French stocks.

Investors fear Le Pen's anti-EU policies could lead to a break-up of the bloc and its single currency, but they are following polls which have shown that of all her main opponents, Macron has the largest predicted winning margin over her.

Macron, a centrist ex-banker, took to Twitter to deride the National Front leader, whose fishing boat outing in jeans and a white jacket won her extensive TV coverage for a second straight day.

Flanked by fans and fishermen in the Port de Grau port west of Marseille, Le Pen told a horde of journalists on the quayside that she would defend all seafarers and all endangered sectors against invasive European Union regulations.

Hitting out at Macron, she said: "Let me warn you, that man will destroy our entire social and economic structure."

The independent centrist, a 39-year-old who did a stint as a minister in the outgoing Socialist government before breaking away to launch a cross-partisan political movement, mocked his 48-year-old foe in turn on the Twitter social network.

"Madame Le Pen is gone fishing. Enjoy the outing. The exit from Europe that she is proposing will spell the end of French fisheries," he said.

The skirmishing has intensified with the countdown to May 7.

It took a spectacular turn in front of TV cameras on Wednesday when Le Pen paid a surprise visit to a doomed tumble-drier plant in her opponent's home town and promised to save it just as Macron was in a meeting with labour representatives behind closed-doors nearby.

She took selfies with people at the Whirlpool site as he was trying to explain to worker representatives that the company's decision to relocate production in Poland was not something the French state could block.

Macron later went to the site himself, and although Macron held his ground and the tension eventually eased, television channels repeatedly broadcast footage of the candidate being heckled, marking a stark contrast with coverage of Le Pen posing for photographs with workers.

She said in a statement on Wednesday evening that, if elected, she would not let the factory be shut down as planned in 2018 and would bring it under state control for a time if necessary to secure its future.