Protesters on 'Reclaim Power' Day Set to Face Zero-Tolerance Approach by Police Chris Green and Ben Ferguson

One thousand of them have already been arrested, and tomorrow will bring protesters' largest, angriest demonstration in the Danish capital – right outside the building where the crucial negotiations are underway.

Mass arrests are likely as a predicted 5,000 demonstrators descend on the Bella Centre on the same day as many world leaders are scheduled to arrive.

The hardline climate-change activists have come prepared with maps, bicycles – and swimming goggles, to protect against police use of tear gas and pepper spray. The event is being billed as "Reclaim Power" day, threatening both the security of the building and the fragile state of the talks within.

The activists have divided into several groups co-ordinated by Climate Justice Action. One group has been awarded an official permit allowing them to demonstrate just outside the site, while another will remain mobile. Still more protesters will form "bike blocs", arriving on contraptions made from reclaimed bicycles welded together. All will be provided with detailed maps of road blocks and precise instructions on where to go. As at the G20 demonstrations in London in March, protesters have signed up to a text message service allowing them to change their plans at short notice.

This time the demonstration is likely to include members of the extreme fringes of the green movement – many of the NGOs which took part in the G20 demos, such as Greenpeace, have their own tables in the Bella Centre.

Richard Bernard, a spokesman for Climate Justice Action, said: "It's going to be a day where we really disrupt the summit and say 'You're not talking any more, we're talking now,' because we are the people who are affected by this. We don't wish to be violent in any way, but we do want to get our voices heard."

The group said they intended the demonstration to be "non-violent but confrontational", admitting that there was no way they could prevent activists who might have different ideas from attending.

The demonstration will put added pressure on the conference organisers, who have been struggling to cope with the sheer number of people descending on the site each day. Yesterday, hundreds of delegates, NGO representatives and journalists were left standing outside in freezing temperatures for up to nine hours after the UN's accreditation system broke down.

Police have adopted a zero-tolerance approach to the activists, handcuffing anyone who commits minor misdemeanour and sitting them on the street for at least an hour. Yesterday morning, they persevered with this pre-emptive policy, pulling over cars and bicycles on the main routes into the city and searching them.

A police spokesman said: "We're stopping people and removing things that could be used as protective gear in protests, such as gasmasks and shields. We want to stop them before they reach the targets. If you're bringing with you protection gear then your intentions aren't peaceful."

As four people appeared in court yesterday for offences committed over the weekend – two for assaulting public servants, one for disturbing the peace and another for trying to free an arrested person – any hope that the police might relax their tactics was extinguished.

Lars Borg of Copenhagen Police said that his force were prepared "24 hours a day" for anything the demonstrators might be planning.

"We know that some will try to get into the Bella Centre... but we are very prepared. They do not have a chance."

"We will keep up the pressure on the violent activists. As long as they are peaceful, they can do anything they want. But we are also ready to use force if necessary."