France's Marine Le Pen on Sunday invoked Martin Luther King's struggle for civil rights as her far-right party planned to rally its supporters after she was convicted of embezzlement and banned from public office.

The bombshell judgement that could crush Le Pen's dream of winning the French presidency in 2027 has stunned the country's political establishment.

Some leftwing forces and the centrist camp staged counter gatherings on Sunday against the far right, while the judges who convicted Le Pen have received threats.

On Sunday, ahead of the rally, Le Pen urged her supporters to take inspiration from one of America's pre-eminent advocates of nonviolence in the fight for equal rights for black Americans.

"We will follow the example of Martin Luther King, who defended civil rights," she told members of Italy's hard-right League party, who were meeting in Florence, via video-link.

"Thank you, Marine, we don't want to take up any more of your time," said Italian deputy prime minister, League leader Matteo Salvini.

"Today will be an important day for you, for France," he added.

At a meeting of President Emmanuel Macron's Renaissance party in the northern working-class Paris suburb of Saint-Denis, former prime minister Gabriel Attal accused the far right of "attacking our judges, attacking our institutions".

"We, here, will never disqualify a court decision," said 36-year-old Attal, speaking in the presence of Prime Minister Francois Bayrou and fellow former premier Edouard Philippe, who also hopes to run in the 2027 presidential elections.

On Monday, Le Pen, 56, was found guilty of embezzling European Parliament funds and given a partly suspended jail term and an immediate ban on holding public office.

Her supporters branded the ruling politically motivated, but Macron insisted the French judiciary is "independent".

US President Donald Trump called the sentence a "witch hunt" by "European leftists using lawfare to silence free speech, and censor their political opponent".

Some left-wingers including members of the hard-left France Unbowed (LFI) party staged a counter rally in Place de la Republique which attracted several hundred people. (AFP)