I'm not going to give up, said Le Pen in a speech to her flag-waving supporters with the golden dome of Hôtel des Invalides glittering in the sun behind her.
She called the verdict that fell on Monday a "political decision" and a "witch hunt" against the party.
Before the demonstrations, Le Pen urged her followers to draw inspiration from American civil rights activist King.
We will follow Martin Luther King's example, she said in a video link when she spoke to the Italian sister party Lega in Florence.
The crowd numbered around 10,000, according to Jordan Bardella, the leader of the National Rally party. They chanted "Marine president" and "they must not steal 2027", referring to the next presidential election.
"Trump-like turn"
While Le Pen supporters gathered near the parliament, a counter-demonstration was held on Place de la République on Sunday afternoon. The organizers accuse the National Rally of having made a "Trump-like turn", referring to the US president's constant questioning of the judiciary.
According to Manuel Bompard of the left-wing party Unsubmissive France, over 15,000 people had gathered at Place de la République.
The far-right is a dangerous party, dangerous for democracy and dangerous for the rule of law, he said to journalists.
At a meeting with President Emmanuel Macron's party Renaissance in the Paris suburb of Saint-Denis, former Prime Minister Gabriel Attal accused the far-right of "attacking judges, attacking our institutions".
He also commented on Le Pen's comparison of herself to Russian regime critic Alexei Navalny, who died under unclear circumstances in a penal colony over a year ago.
You are not Navalny, you are one of his hangmen's assistants, said Attal.
Barred from elections
Le Pen and around 20 other party representatives for the National Rally have been convicted of embezzling EU funds. Her sentence was four years in prison, half of which was suspended, and an immediate ban on running in political elections for the next five years.
She has appealed and has a chance to run if the verdict is overturned or changed.
According to French media, the judges who convicted her have been threatened.
