At about 2pm, they started to stream down corridors and sift down escalators, weaving through the halls leading to the negotiating rooms and out through the main security gate - hundreds of them.
The United Nations climate change talks had not been going well - perhaps an understatement - and for the first time in the 18-year history of these negotiations, the environment groups and civil society organisations had had enough.
It was time to walk out of Poland's national sports stadium in Warsaw - groups from all corners of the world donning white t-shirts and streaming out of the talks, known as COP19, beneath a giant United Nations sign reading "Welcome".
Speaking exclusively to DeSmogBlog outside the talks after checking in his UNFCCC badge, Kumi Naidoo, the human rights activist and executive director of Greenpeace International, insisted the walkout was not a sign of them giving up.
"It is clear that what's happening here is not just betrayal to future generations - because it is clear that the people negotiating here are not going to bear the brunt of climate impacts. It's our children and grandchildren that are going to.
"This is not about giving up, but is about taking the struggle to a different level. If we are to get a solution out of this COP we need people around the world to start - in every country - putting pressure on their governments to actually come to these COPs with a very strong mandate which has serious levels of ambition with regards to cutting carbon.
"Serious ambition too with regard to ensuring that poor countries have the money to adapt to climate change and not follow the same dirty energy pathway that rich countries followed to build their economies."
Kumi Naidoo begins to walk out of COP19 after addressing the media. Photo credit: Brendan DeMelle, DeSmogBlog
"Essentially what we are pushing for here comes in two messages. One is to the negotiators to get your acts together - you cannot continue to drag your feet and understand that nature does not negotiate. Stop playing political poker with the future of the planet. Stop thinking that you can change the science. You can't change the science - you have to change the politics.
"Our second message is to the global public. We have been coming here year after year.We have been trying our best. But our best is not actually delivering and, in fact, the problem with this COP - and this is a statement about this very COP and is not an attack on the UN."
Groups had been outraged at their hosts, Poland's Government, for also agreeing to host a two-day international coal summit at the beginning of the second week of the talks, which should conclude Friday. The main aim of the talks was to make a solid start on a new global deal to cut emissions which could be signed at COP21 in Paris in 2015.
Naidoo said the frustration was directed at the Warsaw talks and not the United Nations Framework Convention on Climate Change process as a whole. He promised that groups would return to COP20 in Lima, Peru, next year.
"The UNFCCC can do their best, but if the countries that are the negotiating parties come with such low levels of ambition then we can't make progress. So it is really a message to the people that you have got to get mobilised. Increasingly I say to people who are parents and grandparents - understand that this struggle is for our children' and grandchildren's future.
"No - I think the UNFCCC will understand. My sense is that Christiana Figueres (UNFCCC executive secretary) is somebody who understands urgency. I have spoken in very many conferences where she gets how urgent things are and she will understand. This was necessary as a wake up call to our negotiators here but also as an appeal to the people around the world - you have to get involved. We cannot leave it to the folks sitting here. If they don't feel pressure - if they don't feel resistance - in each of the different countries then we don't actually get too far."
Maciek Muskat, office director at Greenpeace Poland, told DeSmogBlog: "At some moment in time, one has to say enough is enough. At the moment it looks like the talks are heading for some kind of fiasco and we don't want to endorse it."
He named Canada, Japan, Australia and Poland as being the key governments who had blocked progress on the talks.
Leading the front of the line of WWF International delegates was Tasneem Essop, of WWF South Africa. As she made her way out, she told DeSmogBlog she felt a combination of "anger and frustration". "Governments have come here with no strong mandate to fight climate change," she said.
"We are frustrated and angry with governments but what is heartening is that we have a broad cross section of movements joining together here to say that enough is enough"
Iain Keith, senior campaigner at Avaaz.org, said his membership organisation would not be joining the walkout. "We are a member-funded and member supported organisation and we do not have the buy in from our members to leave this process," he said," but we support those that are and we share their frustrations."
He said the Polish government had managed the COP meeting "terribly" by inviting the coal industry to have a summit at the same time in the same city. He added: "Inviting coal to be the voice of business at this summit has been a terrible choice and has created a huge amount of bad will at this conference."
Kumi Naidoo addressing the media about the decision by civil society groups to walk out of COP19. Photo credit: Brendan DeMelle, DeSmogBlog