"We are estimating the numbers at some 30,000, but it is only an estimate," a duty officer told AFP.

Danish television channel TV2 News put the figure at between 30,000 and 100,000, the total claimed by one of the organisers, Greenpeace International chief Kumi Naidoo, in a statement to the crowd.

Police had said earlier they expected more than 50,000 participants in the rally, which was to be followed by a six-kilometre (four-mile) march through the city to the Bella Center, the conference venue.

Speakers addressing the crowd before it set off made repeated appeals for calm, after authorities said they feared violence on the part of extremists.

"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," Naidoo said.

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