Emmanuel Macron was booed and jeered by striking factory workers during a chaotic campaign visit in northern France.
The presidential candidate had been meeting with union leaders from the Whirlpool plant in Amiens, where jobs are at risk because the production of tumble dryers is going to be moved to Poland.
But during the closed-doors talks, he was upstaged by his far-right rival Marine Le Pen, who made an unannounced visit to the picket line - taking selfies with blue-collar employees and declaring herself as the candidate of France's workers.
She vowed to stop the factory from closing if elected on 7 May, and told the striking staff: "Everyone knows what side Emmanuel Macron is on - he is on the side of the corporations.
"I am on the workers' side, here in the car park, not in the restaurants of Amiens."
:: Who is Marine Le Pen?
When Mr Macron paid a visit to the same picket line a few hours later, the crowds booed and whistled - with some shouting "President Marine!" in the background.
One woman was heard asking him: "Why didn't you come here before?"
Another man yelled: "Save our jobs, Monsieur Macron!"
The pro-business candidate defended his decision to meet with union leaders before going to the factory, and said he would not ban job losses if elected.
Although Amiens is Mr Macron's hometown, Ms Le Pen got more votes than him there during the first round.
:: Who is Emmanuel Macron?
Ms Le Pen, who is against the European Union, said the Whirlpool plant was a "symbol of odious globalisation, which leads to plants moving abroad, destroying thousands of jobs".
But Mr Macron insisted that re-establishing French borders was "a promise made of lies" which would do more harm than good.
The contrasting images of Ms Le Pen smiling with workers while Mr Macron engaged in heated exchanges with them could prove damaging to his presidential campaign.
The leader of En Marche!, which translates as On The Move, has already faced criticism for what some regarded as a triumphalist speech after he won the first round of voting in the presidential election on Sunday.
Jean-Christophe Cambadelis, who leads the Socialist Party, told French radio: "He was smug. He wrongly thought that it was a done deal."