The prosecutor's office in the Alpine town of Gap told AFP the six were detained for "helping illegal immigrants enter national territory".
The group are accused of helping around 30 migrants cross into France via the Montgenevre Pass on Sunday when around a hundred activists, most of them French and Italian, crossed the border with the group.
The activists and migrants clashed briefly with security forces at the ski station before making it to a migrant aid centre in the nearby French town of Briançon.
The incident came after far-right activists from the pan-European movement Generation Identity blocked the nearby Echelle pass in protest at the rise in migrant crossings of the Alps, as other routes around Europe have been shut down.
The group unveiled a massive banner on the mountain reading "Closed border, you will not make Europe home! No way. Back to your homeland!"
Thousands of young men from francophone west Africa have trudged across the mountains over the past two years, dreaming of jobs in France.
Interior Minister Gerard Collomb said both "far left and far right" groups were responsible for "unacceptable action" on the mountain, hours before lawmakers passed a controversial new immigration law.
The law offers more help to those granted refugee status in integrating in France, but also makes it easier to deport those rejected as "economic" migrants.
Rights groups object to provisions that allow failed asylum-seekers to be detained for up to 90 days pending deportation and a reduction of the time in which people can appeal a rejected claim to just two weeks.