Ten Greenpeace activists scaled Barcelona's landmark Sagrada Familia basilica Friday to demand the release of 30 people jailed in Russia for taking part in an protest by the environmental group against Arctic oil drilling.

The activists, many wearing green T-shirts, hung banners from the spires of Antoni Gaudi's unfinished masterpiece with photos of the jailed protesters along with banners that read "Freedom" in English, Spanish and Catalan.

They remained perched about halfway up the spires of the basilica, which rise to over one hundred metres (300 feet), for about three hours while other activists handed out information on the arrest of the protesters in Russia to onlookers on the ground.

"We chose the Sagrada Familia because it is known throughout the world. We want to transmit our message of freedom for our colleagues to the whole world," Greenpeace spokesman Luis Ferreirim told reporters.

Russian coastguards boarded the Dutch-flagged Arctic Sunrise icebreaker on September 19 and arrested its crew of 28 Greenpeace activists and two journalists after they tried to scale a Gazprom oil platform off Russia's northern coast, the country's first offshore oil platform in the Arctic.

The 30 crew members, who come from 18 countries, were initially charged with piracy and were placed in pre-trial detention in Russia until November 24. Several have complained about their conditions, including being kept in isolation, cold cells and lack of adequate food and clothing.

Russian investigators last month reduced the piracy charge, which carries a maximum 15-year prison term, to hooliganism -- the same charge used against the Pussy Riot punk band for a protest performance against Putin.

The Pussy Riot action in a Moscow cathedral in February 2012 landed two band members in prison for two years.

Greenpeace has said the Arctic activists never received official papers formally lifting the piracy charge.

An international maritime court in the German city of Hamburg began hearing on Wednesday a Dutch complaint over Russia's detention of the Netherlands-flagged ship, the Arctic Sunrise. Moscow boycotted the hearing.