Kumi Naidoo of the Amsterdam-based organization delivered the letter to the Russian Embassy in The Hague, Netherlands on Wednesday.
He said he would guarantee the activists' good conduct but said that piracy charges against them don't make sense and should be dropped.
A group of 28 Greenpeace activists and two journalists have been held since their ship, the Arctic Sunrise was seized by the Russian Coast Guard after a protest outside a Gazprom-owned oil rig Sept. 18.
Last week they were charged with piracy, which carries a sentence of up to 15 years.
The Canadians being detained are Alexandre Paul of Montreal and Paul Ruzycki of Port Colborne, Ont.
_ With files from The Canadian Press.