Tens of thousands of people were marching to urge those meeting in Copenhagen to come to some sort of binding agreement to curb global warming.
Police made 968 arrests, including about 400 members of militant groups from across Europe known as Black Blocs. About 150 were released after questioning.
Demonstrations were also held in Australia, Indonesia and the Philippines demanding tough measures by the 194 nations gathered at the UN Framework Convention on Climate Change (UNFCCC) conference.
The summit is due to end on Friday, Dec 18, with a gathering of more than 110 heads of state and government to seal a deal committing major economies to curb emissions of heat-trapping fossil-fuel gases.