Tens of thousands of protesters marched through the chilly Danish capital and 600 were detained Saturday, in a mass rally to demand an ambitious global climate pact just as talks hit a snag over rich nations' demands on China and other emerging economies. The mostly peaceful demonstrations in Copenhagen provided the centerpiece of a day of global climate activism stretching from Europe to Asia. Police assigned extra officers to watch protesters marching toward the suburban conference center to demand that leaders act now to fight climate change. Police estimated their numbers at 40,000, while organizers said as many as 100,000 had joined the march from downtown Copenhagen.