The head of Greenpeace Spain, freed from 20 days in Danish custody along with three others, on Thursday accused authorities in Copenhagen of making them "pay" for the failure of the climate summit.

Juan 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 in a limousine and wearing evening attire.

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

They were arrested along with two others, Christian Schmutz of Switzerland, who posed as their bodyguard, and Joris Thijssen of The Netherlands. All four were released on Wednesday.

"We were made to pay for the anger of the Danish government after the problems of the Copenhagen summit," Lopez de Uralde told Spanish National Radio.

He also denounced their treatment in prison.

"We were very shocked by the way in which we were treated," he said. "We did not expect to get off scot-free... We were taking a risk. What was completely disproportionate was the 20 days in isolation in prison, without knowing the extent of the charges against us, without speaking to anyone."

He rejected suggestions that the isolation was necessary as part of the police investigation.

Police "did not question us until the last day. If it was really for the purposes of the investigation, why not question us on the first day?" he said.

The four have been charged with illegally entering state property, among other things. They still face trial and possible prison sentences.

Their release came a day in advance of their detention being reviewed by a Danish judge.

Lopez de Uralde thanked supporters for the pressure they put on Danish authorities to release him and the three other activists when he arrived at Madrid's Barajas airport and said he had no regrets.

"If it was not for all the support which we received, I think the inentention of the Danish authorities was to keep us in jail for a long time. I don't regret anything, on the contrary," he told reporters.

"I think if I had the same opportunity, at such an important moment, when there were 120 world leaders gathered at the same place, I would do it again to try to pass on a message urging them to do something about climate change. All four of us are very proud of what we did."

The two-week UN climate conference in December produced a non-binding agreement, leaving many observers skeptical about the possibility of limiting global warming to levels considered acceptable.