Tens of thousands of protesters marched through Copenhagen on Saturday calling for tough action from the UN climate conference, after police arrested some 400 rioters at the start of the protest.

Organisers of the rally had repeatedly urged the crowd to remain calm and friendly before the march began, and the speeches were dominated by calls for social justice and critiques of global capitalism.

Soon after the demonstration started, however, police arrested some 400 protesters, masked youths dressed in black who threw bricks and firecrackers and smashed windows in the city centre.

Around 50 police in riot gear moved in, forcing the protesters to the ground and bundling them into vans.

Most of the march passed off peacefully, however.

One protestor dressed as Santa Claus held up a banner saying warming was occurring twice as fast in the Arctic and that "my Rudolf cannot take it any more."

Other demonstrators sported banners that read: "There is no planet B," "Change the politics not the climate," and "Nature does not compromise".

"Each year 300,000 people are dying because of climate change," Kumi Naidoo, executive director of Greenpeace International, told the marchers. "This is not about adaptation, it is about survival."

"We cannot allow carbon traders to damage the world," added Nigeria's Nnimmo Bassey, chair of Friends of the Earth International.

"There is no such thing as clean coal or clean crude. Leave the oil in the soil, leave the coal in the hole."

A police statement spoke of 400 arrests made and identified the suspects as members of militant groups from northern Europe known as Black Blocs.

They were accused of provoking street violence during a NATO summit in the French city of Strasbourg last April.

Officers had preferred to intervene fairly early "to stop the situation deteriorating" said a spokesman.

Later Saturday, a police officer was injured and four cars were burned out during clashes at a squat in Christiania, in central Copehagen.

The officer was hit in the jaw by a brick. Police arrested around 20 people, said a police statement.

Downtown Copenhagen was in virtual lockdown Saturday with thousands of police deployed or on standby and helicopters hovering overhead.

The six-kilometre (four-mile) march by environmentalists and anti-capitalist demonstrators through the city centre followed a route towards the venue of the ongoing UN conference.

"We put their number at around 30,000 but this is just an estimate," a police official told AFP. The Danish TV2 News channel estimated between 30,000 and 100,000.

Earlier, police had warned demonstrators to remain peaceful. "They must not cross certain limits," said police second in command Per Larsen.

The authorities had already deported two Britons for vandalism and spitting on a police officer and a Frenchman for breaching firearms laws, police commissioner Lars Christian Borg told AFP.

They also confiscated a mixture of cooking oil and diesel from a French green group advocating the use of vegetable oil to run vehicles: officers argued it could be used for making bombs.

Within the Bella Center congress hall, Nobel Peace Prize winner Archbishop Desmond Tutu was to lead children in creating "a sea of candles" representing a call from generations imperilled by climate change.

Due to address the crowd outside were Danish supermodel Helena Christensen, west African singer Angelique Kidjo and Mary Robinson, who has served both as the UN high commissioner for human rights and as Ireland's president.

Australia, the developed world's worst per capita polluter, kicked off the worldwide demonstrations -- marches are expected in 130 different cities -- with up to 50,000 people taking to the streets nationwide, said 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.