The rioters went on the rampage in the heart of the Danish capital, prompting swift arrests as some 50 policemen in riot gear intervened, news agency AFP reports.
The rest of the demonstration remained mostly peaceful. The city was taken over by environmentalists and anti-capitalist demonstrators on a march that took them to the venue of the ongoing UN conference. The estimated number of participants ranges from 30,000 to 100,000.
Danish police had issued a strong warning to potential rioters. The authorities were in close contact with their Swedish counterparts to prevent possible rioters from crossing the boarder.