COPENHAGEN (Reuters) - Police detained nearly 1,000 people in Copenhagen on Saturday during mass demonstrations to demand that negotiators at U.N. talks agree a strong treaty to fight global warming.

Tens of thousands of people marched through the city as part of a global "Day of Action" of climate rallies from Australia to the United States, but violence flared at one stage when demonstrators smashed windows and set fire to cars.

Riot police detained more than 900 people around the Danish capital after black-clad activists threw bottles and smashed windows. A police spokeswoman said the number had climbed to 968 shortly after 10 p.m. (2100 GMT).

Police said four cars were set on fire during the evening. One policeman was hurt by a stone and a Swedish man injured by a firework.

"You don't have to use that kind of violence to be heard," said Connie Hedegaard, the Danish minister presiding at the U.N. talks. She condemned rioters after welcoming the main march at a candlelit vigil outside the conference centre.

One activist group accused the police of abuse after they detained around 400 black-clad demonstrators at the back of the march and forced them to sit on a road for hours in near-freezing temperatures, hands bound behind their backs.

The main demonstration was led by dancers, drummers and banners proclaiming: "There is no planet B" and "Change the politics, not the climate". Some activists were dressed as penguins with signs reading: "Save the Humans!"

They marched to the conference centre on the outskirts of the city, where negotiators from 192 nations are meeting from December 7-18 hoping to agree a new U.N. climate pact.