Three people yesterday appeared in a French court on charges of illegally assisting a group of migrants trying to cross into the country from Italy, a case that has fuelled protests that prosecutors are punishing a "crime of solidarity".

Eleonara Laterza, a 27-year-old Italian student; Bastian Stauffer, 26, a Swiss student; and Theo Buckmaster, a 23-year-old Swiss-Belgian, were detained for 10 days in April after participating in a march of 100 activists to escort some 20 migrants over an Alpine pass.

They were responding to a blockade set up by several dozen members of a far-right group, Generation Identitaire (Identity Generation), at a nearby pass to keep them out.

The trial is the latest involving activists trying to aid migrants on the move through Europe, in protest at a tough new immigration law being pushed by French President Emmanuel Macron.

About 120 leading French education, scientific and political figures signed a tribune in French daily Le Monde this week supporting the three activists, accusing prosecutors of flouting France's constitutional promise of fraternity and equality for all.

Several dozen protesters gathered outside the courthouse in Gap, southeast France, as the proceedings opened to stage a "reverse trial" against the government.

"We're accusing the state and the government's migrant policies," said Michel Rousseau of the Tous Migrants (We're All Migrants) association.

Lawyers for the three told AFP they would seek to postpone the hearing while awaiting a ruling from France's top Constitutional Court on whether aid to illegal immigrants should be considered a criminal offence.

They also hope to ease the terms of their conditional release from jail, so they can pass end-of-term exams or return to their jobs.

The activists risk up to 10 years in prison and fines of up to 750,000 euros ($880,000), as well as a ban on entering France.