Twenty-eight activists and two journalists were arrested at the end of September during their marine protest against Arctic oil drilling.
Russia says they will go on trial for hooliganism with some possibly facing charges of refusal to comply with an official order.
Greenpeace has called on six French companies doing business in or with Russia, including oil giant Total and the BNP-Paribas bank, to intervene.
The organisation says these companies have direct contact with, or investments, with Gazprom, the Russian oil company, which is putting pressure on the Russian government to keep the activists in detention.
All the companies have committed to protecting human rights, as part of their corporate responsibility programmes and all but one have signed the UN Global Compact, which commits them to respect human rights where they do business, Greenpeace France campaigner Frederic Amiel told RFI.
French Prime Minister Jean-Marc Ayrault has appealed to Russian President Vladimir Putin to make a "humanitarian gesture" in favour of French national Francesco Pisanu.