The activists' ship was seized and towed to shore on Tuesday after two of the group tried to scale the platform to protest against Russian plans to drill for oil in the Arctic, which they say pose a threat to the fragile ecosystem.
The 30 activists, who were onboard the ship, were due to be questioned yesterday, Greenpeace said, a day after Russian investigators said they had opened a criminal case on suspicion of piracy, a crime punishable by up to 15 years in jail.
Speaking at an Arctic forum, Mr Putin said: "It is absolutely evident that they are, of course, not pirates, but formally they were trying to seize this platform. It is evident those people violated international law."
Russia is concerned such protests could undermine efforts to draw foreign investment to its efforts to tap rich resources in the Arctic.
Greenpeace said the detentions violated international law. Russia has said the protest at the Gazprom-owned oil platform violated the country's sovereignty.