"Several hundred people are now corralled by a very stiff Danish police presence - riot police in full gear wielding batons," he said.
"A little while ago there were a few critical moments as they tried to breach that barricade. But since then the crowd has been compressed. It's smaller now in number and it is encircled by the police."
Police said some 230 protesters had been detained during Wednesday's action. There were no reports of injuries.
But Climate Justice Action, which organised the march, said demonstrators were determined to get past the barricade.
"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.
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 talks as they enter their final stretch.
"I think there was a hope that somehow through the discussions both sides would come together - that clearly hasn't happened"
Alan Fisher, Al Jazeera correspondent
Negotiations so far have been hampered by divisions between rich and poor nations over cuts to greenhouse gas emissions and financing for developing nations.
Alan Fisher, Al Jazeera's correspondent inside the summit, said talks have entered a crucial point, with hope resting on political heavyweights breaking the deadlock.
"Everyone knew there were differences between the rich developed nations and the poor developing nations before this conference even got underway.
"I think there was a hope that somehow through the discussions both sides would come together - that clearly hasn't happened," he said.
"Now the political heavyweights are getting involved trying to make sure that these talks stay on track."
He said Wednesday's replacement of Connie Hedegaard, the Danish president of the summit, with Lars Lokke Rasmussen, the country's prime minister, was a sign that stronger action was needed.
Hedegaard told Wednesday's meeting: "With so many heads of state and government having arrived it's appropriate that the prime minister of Denmark presides".
Hedegaard, who will continue to participate in negotiations, said the move was procedural.
Barack Obama, the US president, and Wen Jiabao, the Chinese prime minister, are expected to arrive in the last few days of talks.