"We will get past the police cordon so that we can hold a popular assembly and discuss with delegates from the summit ... to get a climate solution," Peter Nielsen, the group's spokesman, said on Danish TV2 news.
"The police have tried to get in our way all week now. This is a question of resolving a global problem, and we will not hold people back," he said.
Johnny Lundberg, a police spokesman, told the AP news agency that activists were arrested on Wednesday for attempting to pass through the security perimeter set up around the suburban conference centre.
There were no reports of injuries.
Security in the Danish capital has been beefed up to try and stop activists from accessing the Bella Centre, where representatives from 193 countries are negotiating a new global warming deal.
World leaders have begun arriving at the 12-day talks as they enter their final stretch.
Barack Obama, the US president, and Wen Jiabao, the Chinese prime minister, are expected to arrive in the last few days of talks.
Connie Hedegaard, the Danish president of the summit, has stepped down from her role as talks come to a head, with Lars Lokke Rasmussen, the country's prime minister, taking her place.
"With so many heads of state and government having arrived it's appropriate that the Prime Minister of Denmark presides," she told Wednesday's meeting.
Hedegaard, who will continue to participate in negotiations, said the move was procedural.