Some 300 youths shrouded in black threw bricks and smashed windows as at least 30,000 people demonstrated in central Copenhagen on Saturday to turn up the heat on world leaders debating global warming.
The rioters, whose faces were also covered, went on the rampage in the heart of the city, prompting swift arrests as some 50 policemen in riot gear intervened.
Demonstrators were forced to the ground and then bundled into vans, say witnesses.
The rest of the march -- the centrepiece of protests planned in 130 cities across the world -- remained peaceful although tension was building as protestors condemned the violence.
The heart of the Danish capital was taken over by environmentalists and anti-capitalist demonstrators on a six-kilometre march that would take them to the venue of the ongoing UN conference.
"We put their number at around 30,000 but this is just an estimate," a police official on duty told AFP. The Danish TV2 News channel put the crowd strength between 30,000 and 100,000.
Organisers of the rally had repeatedly asked the crowd to remain calm and friendly before the march began, dominated by calls for social justice and against capitalism.
"Copenhagen is in the eye of the storm. Each year 300,000 people are dying because of climate change. This is not about adaptation, it is about survival," Kumi Naidoo, executive director of Greenpeace International, said in a speech.