"Don't believe everything that is written in this blueprint", Jon Palais, co-founder of the NGO Alternatiba said on Saturday, on the sidelines of an anti-Cop21 demonstration near the Eiffel Tower.
"The Paris Climate Conference has betrayed its mission of stabilizing climate change," he insisted, warning citizens not to be fooled by leaders' "nice speeches".
"We've heard fancy rhetoric coming out from government leaders for an entire year, but this hasn't prevented the level of greenhouse gases from increasing.
Hours beforehand, French President François Hollande vaunted the "historic moment" the world was entering, as 195 nations mulled over whether or not to endorse the Paris' climate deal.
Talking of a "major act for humanity," the French leader called on delegates gathered at the climate conference to make "December 12, not only a historic day, but a date for humanity."
Yet for thousands of environmental activists demonstrating on Saturday, the COP21 agreement could instead 'allow a global crime against humanity,' unless radical changes are implemented, a coalition of NGOs stated in their tract.
Forming a human chain, brandishing red lines and holding a minute of silence, they marked in their own special way, the end of the Paris talks, which wrap up a week of tough negotiations.
Whilst they urge citizens to remain vigilant and mobilize against climate change because leaders 'have failed to', other environmental groups like Greenpeace gave more positive reactions to the Paris' climate-saving blueprint.
"The accord deals a major blow to the fossil fuel industry," Greenpeace's international executive director Kumi Naidoo told AFP.
By striving to limit global warming to 1.5°C (2.7 degrees Fahrenheit) over pre-Industrial Revolution temperatures, "the accord is crucial," Naidoo said.
"That single number, and the new goal of net zero emissions by the second half of this century, will cause consternation in the boardrooms of coal companies and the palaces of oil-exporting states," Naidoo predicted.
The accord still needs to be endorsed by 195 countries.