The director of Greenpeace Spain has complained of being "treated like a dog" at a Danish prison where he has been held since his arrest at the UN climate summit last month, Spanish media reported Tuesday.

The online editions of daily newspapers El Pais and El Mundo said Juan Lopez de Uralde told family members who visited him on Tuesday at the Vestre Faengsel jail for the first time since his arrest that he was being treated poorly.

"They are trying to humiliate us, to break us. They treat us like dogs," the papers quoted him as telling his wife and brother during the visit.

In a statement, Greenpeace Spain said Lopez de Uralde had lost weight while in prison and he spent his first 24 hours in the jail in a cell with 60 other inmates that had only a few mattresses on the floor.

His visit with his wife and brother was monitored by a policeman and a translator and since Monday Lopez de Uralde and the three other activists who were detained along with him have "been mixed with common criminals", it added.

"The measures being taken with them seem to us to be disproportionate and we believe they are a form of punishment against civil society which fights for a better world," the campaign director of Greenpeace Spain, Mario Rodriguez Vargas, said in the statement.

Lopez de Uralde, and Norwegian Nora Christiansen 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 the incident.

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