[link to news.sky.com]

Hundreds of youths dressed in black threw bricks and smashed windows as at least 30,000 people demonstrated in the centre of the city as world leaders debate global warming.

The rioters, whose faces were 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.

Police said those arrested were members of militant groups from northern Europe known as Black Blocs, who were accused of provoking street violence during a Nato summit in the French city of Strasbourg last April.

The rest of the march - the centrepiece of protests planned in 130 cities across the world - remained peaceful, ahead of a candlelit vigil.

Organisers say around 100,000 people have been taking part. Police put the number at nearer 30,000.

Earlier, two Britons were deported for vandalism and spitting on a police officer during the protests.