"The establishment has dropped a nuclear bomb. If they're resorting to such extreme measures, it's because we are on the verge of winning," Le Pen said in Paris on Tuesday, referring to the presidential election in 2027.
She accused her rivals of escalating their efforts after previous attempts to undermine her had failed.
Le Pen was seen as a front-runner in the race to replace Emmanuel Macron, who cannot stand for a third term.
Le Pen, 56, was found guilty of embezzling European Union funds and sentenced to a five-year ban from running for office on Monday.
The ruling takes immediate effect.
Le Pen was also given a four-year prison sentence but it will only begin once no further legal recourse remains.
Two years are to be served wearing an electronic monitor while the two further years were suspended.
Le Pen, who is one of France's best-known politicians, said she would continue to fight: "This is about defending the French people, who have the right to choose their leaders, and about protecting a nation that is faltering."
Calling the court's decision "insane," she vowed to challenge it.
Her political protégé, National Rally party leader Jordan Bardella, urged supporters to resist "peacefully and democratically" what he called a grave injustice.
"Nothing will stop us from fighting to take power," he declared, adding that the party would explore all legal options to overturn the ruling.
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