Wearing "Polluters talk, we walk" T-shirts, the activists streamed out of Warsaw's National Stadium, where rich and poor countries were arguing over who should do what to fight global warming.
The two-week session in the Polish capital was never expected to produce any big decisions or breakthroughs, but the protesters said in a statement that the talks were "on track to deliver virtually nothing."
Negotiations have been bogged down by disputes over financing to help poor countries develop their economies in a cleaner way than the West did and cope with rising sea levels, desertification and other impacts of global warming.
Meanwhile, emerging economies including China and Brazil appeared to resist a European push for setting a 2014 deadline for when countries should put forth commitments for a new climate agreement, which is supposed to be adopted a year later.
The level of progress is seen as a possible indicator of the world's chances of reaching a deal in 2015. That's the new watershed year in the U.N.-led process after a 2009 summit in Copenhagen ended in discord.
"If we go with the spirit of the lack of urgency that we see in these talks, we are headed for another disaster in Paris in 2015 and we need to avert it at all cost," Greenpeace head Kumi Naidoo told The Associated Press.
"We cannot afford to get it wrong again in 2015," he said.
Environmental groups from around the world attend the annual talks as observers. They often stage colorful protests in the hallways to urge negotiators to step up the pace. One group called the Climate Action Network names a "fossil of the day" in a daily mock award ceremony to shame countries they see as blocking the negotiations.
In Warsaw, some activists joined Filipino delegate Naderev Sano's fast during the conference in support of the victims of Typhoon Haiyan. Sano said his delegation shared the protesters' "anger and frustration" at the lack of "political will" at the negotiations.
Poland's Marcin Korolec, the president of the talks, said he didn't share the climate activists' assessment of the negotiations.
"We have achieved considerable progress on climate finance," Korolec said. "The talks about the shape of a new global agreement were also held throughout the night. I am convinced that we are getting closer and closer to a final success."
Some activists rejected the walkout.
"I share the anger of the NGOs that walked out and agree with the spirit of what they have done," said Mohamed Adow, a senior climate change adviser at Christian Aid. "But I am committed to this process, which is not at fault, and want to be here to fight for what can be achieved in these last hours and to hold the rich countries to account."
The U.N. climate talks were launched in 1992 following scientific warnings that humans were warming the planet by emissions of CO2 and other greenhouse gases. The next round of the annual talks is set to be held in Lima, Peru.