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, an AFP reporter witnessed.
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 reporter said.
The heart of the Danish capital was taken over by environmentalists and anti-capitalist demonstrators on a six-kilometre (four-mile) 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 urged 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.
Nigerian Nnimmo Bassey, chair of Friends of the Earth International, said, "We cannot allow carbon traders to damage the world. There is no such thing as clean coal or clean crude. Leave the oil in the soil, leave the coal in the hole."
The demonstrators included a group from China sponsored by businessman James Hong, a designer and clothing manufacturer from Fujian province, who paid out of his own pocket for 15 of his employees to come to the Danish capital.
Central Copenhagen was in virtual lockdown, with thousands of police deployed or on standby ahead of the march.
Other demonstrators sported banners that read: "There is no planet B," "Change the politics not the climate," and "Nature does not compromise".
A thorny issue was clearly the responsibility of the bigger polluters to do more to arrest climate change.
Others meanwhile said the haves should not be let off the hook by being allowed to buy carbon credits from other countries.
"It's a question of justice, we must cut carbon emissions at home instead of buying carbon credits elsewhere," said Susann Scherbarth, a 28-year-old German activist from Friends of the Earth.
Jakob Larsen, 22, of Denmark said: "Capitalism is not the right way to go if you want to save the planet. Climate change has come about because capitalism doesn't care."
Some Asian protestors pointed an accusatory finger at themselves.
Lee Sung-jo, climate coordinator of the Korea Federation for Environmental Movements, said: "Korea has a responsibility to reduce its CO2. We are not a developing country any more."
Danish police had issued a strong warning to potential rioters.
"They must not cross certain limits," said police second in command Per Larsen.
Authorities also expelled offenders, deporting two Britons for vandalism and spitting on a police officer and a Frenchman for breaching firearms laws, police commissioner Lars Christian Borg told AFP.
Within the Bella Center congress hall, Nobel prizewinner Archbishop Desmond Tutu was to lead children in creating "a sea of candles" representing a call from generations imperilled by climate change.
Other celebrities expected to address the crowd include Danish supermodel Helena Christensen, African diva Angelique Kidjo and Mary Robinson, a former UN high commissioner for refugees and Irish president.
Australia, the developed world's worst per capita polluter, kicked off the worldwide demonstrations with up to 50,000 people taking to the streets nationwide, according to organisers.
In Indonesia, third biggest emitter of greenhouse gases after China and the United States, activists rallied outside the US embassy in Jakarta to urge the superpower to support developing nations in reducing emissions.
In the Philippines hundreds of protesters wearing red shirts banged on drums and sang songs outside Manila's City Hall demanding global action on climate change.