'These 30 people are in prison in Russia for staging a peaceful protest in the Arctic! Let your voice be heard. Let's bring these people home!' Madonna wrote in a message posted by Greenpeace on Twitter late Friday.
She spoke out after former Beatle Paul McCartney on Thursday published a letter to President Vladimir Putin saying he hoped the activists could be home for Christmas.
The following day, Oscar-winning French actress Marion Cotillard climbed into a cage outside the Louvre in Paris to call for the activists' release, calling their detention 'absolutely absurd and crazy.'
The activists from Greenpeace's Arctic Sunrise icebreaker are awaiting trial in prisons in Saint Petersburg, where they were moved from the far northern city of Murmansk.
German Chancellor Angela Merkel has already voiced concern over their case while British Prime Minister David Cameron last week urged Putin to treat the so called 'Arctic 30' fairly.
Russia's Investigative Committee said last month it was softening the charges against the crewmembers, who come from 19 different countries, from piracy to hooliganism, which carries a maximum sentence of seven years. But Greenpeace says the piracy charge was never formally lifted.
Madonna has previously used Russian concert performances to speak out on politics. At a concert in Moscow last year she called for the release of Pussy Riot bandmembers who are now serving a two-year jail term for a protest against Putin. She also gave fans pink ribbons at a concert in Saint Petersburg to express her opposition to controversial legislation banning 'propaganda' of homosexuality to minors.
From Agence France-Presse