A judge placed France's far-right leader Marine Le Pen under formal investigation on Thursday on suspicion of disseminating violent images over tweets she posted of Islamic State executions, a move she indicated was politically motivated.

The investigation relates to three graphic images of Islamic State executions she posted on Twitter in December 2015, including the beheading of American journalist James Foley.

"If they are trying to silence me, they will not succeed," Le Pen told news channel LCI. She had denounced an earlier stage of the investigation as "political interference".

If they are trying to silence me, they will not succeed

Under France's legal system, being formally placed under investigation often - but not always - leads to a person being sent to trial.

The judge's decision to step up the investigation comes a little over a week ahead of the National Front's annual congress, where she is expected to lay out how she plans to rebuild the battered party after her thumping defeat at the hands of President Emmanuel Macron in last year's election.

It is, though, unlikely to affect her popularity with party loyalists.

Other ongoing investigations, including one into allegations her party misused European Union funds to pay parliamentary assistants and another into the financing of past election campaigns, have not dented her appeal among the National Front faithful. She has denied any wrongdoing in both cases.

Party stalwarts rallied around Le Pen on social media.

"@MLP_officiel placed under formal investigation for spreading photos of Islamic State executions. No, it's no good telling the truth," tweeted Bryan Masson, a National Front official from the southern Alpes-Maritimes region.

If the investigation proceeds to a trial, Le Pen could face a maximum sentence of several years in jail and a €75,000 fine if convicted.