A Finnish icebreaker contracted to Anglo-Dutch oil giant Shell has left Helsinki for the Arctic despite Greenpeace's attempts to block its departure, the environmental group said Wednesday.

"The ship sailed (Tuesday) evening after our activists were taken away by the police," Veera Juvonen of the Finnish branch of Greenpeace told AFP.

Some 20 activists on Tuesday morning chained themselves to the Nordica icebreaker docked in the Helsinki port, to protest against Shell's plans to drill for oil in the Arctic.

They were removed by police later in the day and detained. A new group of activists arrived on rubber boats in the evening in a bid to prevent the ship from leaving port, and they were also detained.

Police said 40 Greenpeace demonstrators were detained, while Greenpeace said the number was 42.

A Helsinki police spokesman said the last of the protesters was released around midday Wednesday.

The Nordica, owned by the Finnish state, was due to join its sister ship, the Fennica, to support two drill ships en route to the north coast of Alaska to drill five exploratory wells for Shell in the Chukchi and Beaufort Seas respectively, according to Greenpeace.

The Fennica left Helsinki in March.

Juvonen would not disclose whether Greenpeace would try to stop the icebreaker at sea.

"That remains to be seen. I can't disclose such information," she said.

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.