"Overnight, Greenpeace activists held up the Nordica icebreaker for three hours (in waters between Denmark and Germany) by circling the vessel with their rubber speed boats," Therese Jacobson, who is responsible for Arctic issues at Greenpeace, told AFP.
The 14 activists from Sweden, Denmark, Germany and New Zealand then followed the Finnish icebreaker and around 9am caught up with it again.
"They are now painting on its hull," Jacobsen said, adding that so far no attempts had been made to board the vessel again.
The Nordica is under contract to Shell with another icebreaker, the Fennica, to provide support for an operation to drill five exploratory wells in the Chukchi and Beaufort Seas respectively, Greenpeace says.
Some 40 Greenpeace demonstrators were detained Tuesday after they tried to block the Nordic from leaving Helsinki, and on Thursday, Swedish police detained six more activists after they boarded the icebreaker in the middle of the Baltic Sea and chained themselves to the ship.
Nordica's sister ship, the Fennica, left Helsinki in March.
Environmentalists have pointed to the vastly complicated task of drilling in the harsh Arctic environment, the difficulty of effectively cleaning up any spills in such conditions, and the risks posed to wildlife and native communities in the region's fragile ecosystem.
Swedish Greenpeace activists have voiced their criticism of Swedish foreign minister Carl Bildt for not doing enough during Sweden's current presidency of the Arctic Council to prevent drilling for oil in sensitive areas of the Arctic.
In an opinion article published on The Local on Thursday, Bildt argued that a ban on drilling in the Arctic would be "irresponsible".
"Considering how serious the situation is with respect to climate change, it's regrettable that Sweden has a foreign minister who is in favour of drilling for oil in the Arctic," Jacobson said in a statement.
"Do the Moderates and Carl Bildt want Sweden to give up ambitions to save the environment?"