Twenty other activists, including two Canadians, and two journalists were ordered jailed for two months during a marathon court session on Thursday that stretched late into night, but the court ruled to hold the remaining eight only until new hearings could be held on Sunday.
No charges have been brought against any of the activists, who are citizens of 18 countries, including Russia.
Russian prosecutors are considering whether to charge them with piracy, among other offences, and the activists are being held pending the investigation.
The Russian Coast Guard disrupted an attempt on Sept. 18 by two of the activists to scale a platform owned by Russian state-controlled energy giant Gazprom to call attention to the environmental risks of drilling in Arctic waters.
The next day, the Coast Guard seized Greenpeace's ship, the Arctic Sunrise, and towed it to Murmansk with the crew and activists aboard.
Greenpeace Russia campaign director Ivan Blokov has described the seizure of the ship as "the most aggressive and hostile act" against the environmental organization since French government agents bombed the Rainbow Warrior ship in 1985, killing one man.
Peter Wilcox, an American who captained the Rainbow Warrior, also is the captain of the Arctic Sunrise. He was ordered held in custody during Thursday's court session.
Canadian crew member Paul Douglas Ruzycki, of Port Colborne, Ont., was serving as the ship's chief mate. Montrealer Alexandre Paul, 35, was also on board.
Those ordered jailed on Sunday include Dima Litvinov, Greenpeace International spokesman, who has dual U.S. and Swedish citizenship; Finnish activist Sini Saarela, who was one of the two who tried to scale the platform; a British activist; two Dutch citizens and a Ukrainian cook.
The platform, which belongs to Gazprom's oil subsidiary, is the first offshore rig in the Arctic. It was deployed to the vast Prirazlomnoye oil field in the Pechora Sea in 2011, but its launch has been delayed by technological challenges.
Gazprom said this month it was to start pumping oil this year, but no precise date has been set.