The head of Greenpeace International, South Africa's Kumi Naidoo, appealed to Danish authorities Monday to release four members of the organisation arrested at the UN climate summit last month.

The director of Greenpeace Spain, Juan Lopez de Uralde, and Norwegian Nora Christiasen fooled security staff at the Danish parliament in Copenhagen by drawing up to a December 17 gala dinner of the UN climate conference in a limousine and wearing evening attire.

There, they unfurled banners reading "Politicians Talk, Leaders Act" at the entrance.

Two others Greenpeace activists were also held over incident.

Naidoo said he is on a European tour to campaign for their release, as well as to "keep pressure" on the governments following the "disaster" of the Copenhagen summit.

"We are already saddened that the response of Danish authorities has been so disproportionate to keep them in prison during Christmas and New Year, to deprive them visits from their families and easy access to lawyers," he told a news conference in Madrid.

He is to be in Copenhagen on Thursday when a Danish court to announce whether the four will remain in custody.

Greenpeace Spain last week delivered a petition backed by 50,000 people to the Danish embassy in Madrid demanding their release.