The comments came at the beginning of a two-week climate conference in Katowice, Poland, where emissaries from almost 200 nations are meeting to determine how the world can dramatically scale back greenhouse gas emissions to abide by the landmark Paris climate agreement and, by doing so, stave off the worst effects of climate change.
"The upcoming climate talks are the most important round of negotiations since the Paris Agreement was reached three years ago", Lou Leonard, the World Wildlife Fund's senior vice president for climate change and energy, told CNBC of the talks that gather thousands of world leaders, activists, and policymakers to flesh out a framework for the Paris Climate Agreement.
"If we don't take action, the collapse of our civilizations and the extinction of much of the natural world is on the horizon", he said. "Time is running out", he said. "They want you, the decision-makers, to act now".
"Leaders of the world, you must lead". "The continuation of our civilizations and the natural world on which we depend is in your hands".
In 2015, the Paris agreement deal saw nations agree to limit global temperature rises to below two degrees Celsius (3.6 Fahrenheit) and under 1.5C if possible.
Guterres said climate change was already taking its toll.
Thunberg, who protests outside Sweden's parliament each week and has inspired students in other countries, said absent leaders such as U.S. President Donald Trump and German Chancellor Angela Merkel "don't realize how much power they have".
Citing a recent scientific report on the dire consequences of letting average global temperatures rise beyond 1.5 degrees, Mr Guterres urged countries to cut their emissions 45 per cent from 2010 levels by 2030 and aim for net zero emissions by 2050.
Fiji's prime minister, Frank Bainimarama, who presided over last year's United Nations climate summit, said the "just transition" proposal should not just consider the fate of fossil-fuel workers but all people around the world whose lives were affected by climate change.
"In short, we need a complete transformation of our global energy economy, as well as how we manage land and forest resources", Guterres said.
"We feel as if we have been penalised for the mistakes we never made", said Bhandari.
Waskow, who has followed climate talks for years, said despite the Trump administration's refusal to back this global effort the momentum is going in the right direction.
The riots in Paris at the weekend, partly in protest at higher fuel taxes, also illustrate the conundrum: How do political leaders introduce policies that will do long-term good for the environment without inflicting extra costs on voters that may damage their chances of re-election?
Under Paris, richer nations - responsible for the majority of historic greenhouse gas emissions - are expected to contribute funding that developing nations can access to make their economies greener.
"A failure to act now risks pushing us beyond a point of no return with catastrophic consequences for life as we know it", said Amjad Abdulla, chief negotiator for the Alliance of Small Island States, of the United Nations talks.
Attenborough's speech comes at a crossroad for global climate action.
Separately, negotiators will discuss ramping up countries' national emissions targets after 2020, and financial support for poor nations that are struggling to adapt to climate change.
Attenborough, who has produced and narrated numerous nature documentaries, is a strong advocate for fighting climate change - but that wasn't always the case.