Macron was in his hometown of Amiens in northern France to try to counter accusations he had made a complacent start to campaigning after he finished ahead of Le Pen in the first round of the election on Sunday.
But while Macron had arranged to meet workers from the Whirlpool domestic appliance factory without actually visiting the site, Le Pen turned up unannounced and posed for photographs with workers.
"Everyone knows what side Emmanuel Macron is on -- he is on the side of the corporations," Le Pen said.
"I am on the workers' side, here in the car park, not in restaurants in Amiens."
The factory operated by Whirlpool, a US multinational company, is threatened with outsourcing to Poland.
He drew criticism for what some saw as a triumphalist speech and then a celebratory dinner at a Paris bistro on Sunday.
Socialist Party boss Jean-Christophe Cambadelis told French radio: "He was smug. He wrongly thought that it was a done deal."
Macron served as economy minister in the Socialist government before quitting in August to launch his presidential bid.
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