Chris Green: I stood with the protesters as the police batons rained down on them

Eyewitness: The police have behaved abominably. This is a non-violent demonstration

In front of me, a man held his hands high in the air, staring at the riot policeman blocking his way. Suddenly he screamed in pain as a black baton connected with his shoulder. Smack! Another blow rained down, forcing him to stagger back, stunned.

"Oh my God, what are they doing?" shouted a woman to the left. "Somebody get his badge number!" yelled another. Then, all around, a communal chant went up: "Shame on you, shame on you."

The group of protesters was hemmed in on a narrow footbridge, the end blocked by Danish riot police clad in blue protective clothes and helmets. As the crowd gave way under another hail of baton blows, many turned and ran.

They were only just in time. At the other end of the footbridge, two vans screamed to a halt and more blue-suited officers with the word "POLITI" stencilled across their backs leapt out, cutting off the remaining protesters' means of escape.

Those activists left were batoned mercilessly from both sides until they collapsed to the ground. One woman desperately trying to escape stumbled and fell, her face twisted in horror as she disappeared under the stampede.

An hour earlier, the hubbub of the Bella Centre media room had been silenced by the sound of chanting and tramping feet in the corridors. The walk-out promised by the coalition campaign groups Climate Justice Action and Climate Justice Now was taking place, 45 minutes early.

There was no time to grab one's coat before joining them. A group of about 100 people, including Bolivian delegates, strode out of the centre chanting "Reclaim power!" Their aim was to reach the "people's assembly" outside, a larger demonstration intended to give a voice to those who feel let down by the negotiations.

We swept rapidly towards the exit, only slightly hindered by security guards, and were soon out in the freezing Copenhagen air. The police initially allowed the march to progress towards the larger demonstration, but soon blockaded the footbridge and said that anyone who crossed it would be arrested.

Kevin Smith, 36, an activist from London, was on the front line of the protest and was bruised by several baton blows. He said the police's attack had been "totally unprovoked".

"All the people on the bridge were holding their hands up, but the police just started hitting us," he said. "There was just no need for them to do it – they seemed totally out of control. One guy beside me was having his hair pulled by a policeman. Hopefully this will be a wake-up call for Denmark in terms of how policing is handled, like the G20 was for Britain."

The Canadian writer Naomi Klein also joined the walk-out, telling The Independent before the violence started that the march "sent a very powerful message" to politicians.

"People are literally turning their backs on the failed negotiations, which is symbolically very powerful," she said. "I also hope it's emboldening for some of the developing countries, who are hanging on by a thread.

"I think the police have behaved abominably. This is a non-violent demonstration, and I think the police are really escalating it by picking people off in the way that they have – it actually increases the chance of things getting out of control."

Meanwhile, activists outside the Bella Centre were driven back by police using batons, pepper spray and teargas, after they tried to penetrate its perimeter fence. Witnesses said that some activists had been punched in the face by police, and there were injuries.

Some of the activists tried to cross the conference centre's canal using a huge inflatable lilo, but only two succeeded in crossing and were soon apprehended. The "people's assembly" was eventually set up further away from the conference centre, after a total of 260 people had been arrested.

In the morning, a number of prominent campaign groups, including Friends of the Earth, were refused entry to the Bella Centre. They were told their access had been suspended for "security reasons". It is believed that the United Nations Framework Convention on Climate Change (UNFCCC) took the step as a precaution because the groups had taken part in unscheduled protests inside the conference centre earlier in the week.

In response, 80 people staged a sit-down protest at the entrance to the conference centre as delegates filed past. Yvo de Boer, the UNFCCC's executive secretary, came out to talk to them but was shouted down with chants of "Open the door, de Boer!"

He replied: "I'm willing to have a reasonable conversation with you, as I have always done... but if you start abusing me, insulting me and shouting at me, then I'm not interested."

Last night, in an attempt to appease the NGOs and campaigners whose access to the conference has been severely limited, the UNFCCC announced that an alternative venue had been created at Forum Copenhagen near the centre of the city.