Tens of thousands of protesters took to the streets of Copenhagen today to press world leaders for progress on global warming. Estimates ranged from 40,000 (from police) to 100,000 (from organizers). Police say they detained about 600 black-clad youths after bricks were thrown at officers, reports AP. Earlier, about 20 were detained for wearing masks or carrying pocketknives. Today's protest comes at the midway point of the two-week climate negotiations. (More Copenhagen climate change conference stories.)