A high-profile climate activist was arrested ahead of tomorrow's major protests planned outside the Copenhagen climate summit, fuelling anxiety about how the Danish authorities are policing demonstrations.

Arrested activists sit on the ground as they are surrounded by police in Copenhagen. Tadzio Mueller, a spokesman for the umbrella group Climate Justice Action (CJA), was arrested today by plainclothes police as he left the Bella centre, where the official climate talks are taking place. The police refused to say what charges will be brought.(Photograph: Christian Charisius/Reuters) Tadzio Mueller, a spokesman for the umbrella group Climate Justice Action (CJA), was arrested today by plainclothes police as he left the Bella centre, where the official climate talks are taking place. The police are holding him at the Retorvej detention centre, and he will be charged in court tomorrow morning. The police refused to say what charges will be brought.

Kevin Smith, an organiser for activist group Climate Camp, said: "It's unbelievable that in a supposed democracy, undercover police are silencing spokespeople that are criticising the climate talks. How far are the Danish authorities prepared to go to stop tomorrow's protest from going ahead?"

Mueller's arrest comes on the eve of a Reclaim Power action that aims to "disrupt the sessions and open a space inside the UN area to hold a people's assembly" from 10am tomorrow.

For several months, Mueller and other activists from CJA, which includes representatives of organisations from around the world including Via Campesina, Focus on the Global South and Climate Camp, have been planning the protest.

But their plan has now been modified and not all the demonstrators will attempt to enter the centre. The largest section of the demo is likely to be the "blue bloc" which has been granted permission to march by the police. It will gather at Tarnby train station, and walk from there to the north-east point of the fence around the Bella centre. It will be headed by the large groups from the southern hemisphere including Via Campesina and Focus on Global South among others. They will be accompanied by the "yellow bloc", consisting of members of some of the NGOs who have been shut out of the Bella centre to make space for world leaders arriving this week.

From inside, a group of delegates - at least two countries have committed themselves - and some NGOs, will take part in a demonstration of solidarity. According to one organiser, who asked not to be named, there may be acts of civil disobedience. The inside groups hope to be able to come out and join the blue bloc, and hold a "people's summit" inside, or near the perimeter of the centre.

Simultaneously, a "green bloc" and an "autonomous bloc", consisting of various groups, will be coming at the Bella centre from unspecified points and attempting to scale the fence. These groups are reportedly committed to non-violent civil disobedience.

Ed Thompson, a spokesman for CJA, said: "This is potentially a truly historic action, bringing together activists from the north and south, in an attempt to make the voices of the most vulnerable groups who are being left out of these talks heard. We are not intimidated by the police. Every activist I know has come here to take non-violence direct action, and not a single one of them has said that he's going to stay home."

A spokesman for the Danish police said: "For the demonstration tomorrow we're expecting large numbers. We have some information coming in but we will just be monitoring the situation as it carries on. We're hoping, of course, for a peaceful demonstration, where people can exercise their right to demonstrate."

Smith added: "The Danish government knows just how embarrassing it will be when hundreds of delegates walk out tomorrow to join us in the protest tomorrow against the climate talks, and it is trampling over all manner of civil liberties to try and prevent that from happening."