Four Greenpeace protesters arrested for gatecrashing a royal gala dinner for heads of state at the UN climate summit in Copenhagen last month were released from Danish custody on Wednesday, police said.

"They have been charged, but they are free to leave and can leave the country if they want," Copenhagen police duty officer Michael Klausen told AFP.

The four are the director of Greenpeace Spain Juan Lopez de Uralde, Norwegian Nora Christiansen, Christian Schmutz of Switzerland and Joris Thijssen of The Netherlands.

Lopez de Uralde and 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. Schmutz posed as their bodyguard.

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

"You can't detain people, detain them for 20 days and keep them incommunicado and then suddenly say that they are free just like that, with no explanation," Lopez de Uralde told Spanish media after his release, adding he was not allowed to use the telephone or Internet while he was detained.

"The hardest part was the uncertainty, not understanding the process and knowing that there was no reason for such a harsh treatment for what we did," he added.

Lopez de Uralde had earlier complained of being "treated like a dog" at the Danish jail, and Greenpeace Spain last week delivered a petition backed by 50,000 people to the Danish embassy in Madrid demanding their release.

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

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

A Greenpeace spokeswoman told AFP the four were freed after the organisation provided police with the names of other Greenpeace activists who took part in the protest.

In a statement, Mads Christensen, executive director of Greenpeace Nordic, said the imprisonment of the activists "has effectively been punishment without trial".

"The alleged 'crime' is that the four aimed to impress upon world leaders the urgency of acting to prevent catastrophic climate change. The length of their detention without trial is out of all proportion to what was a simple and harmless protest with a legitimate objective," Christensen added.