Citing bleak recent reports, including one from the United Nations expert climate panel in October, Guterres noted devastation from hurricanes in Barbuda and Dominica which he called "heart-breaking", but also "preventable".
Without drastic action, there will be catastrophic consequences, he warned. Guterres called climate change "the most important issue we face".
Duda, the Polish leader, said participants at the conference have backed its proposal of a "just transition" away from coal mining, which calls for helping those people, like coal miners, who are slated to lose their jobs as the world changes its energy mix.
He later told reporters that realities of global climate changes are "worse than expected, but the political will is relatively faded after Paris" and is not matching the current challenges.
Sir David Attenborough encouraged leaders of the world to take a stand and address the issue of climate change while speaking at the UN Climate Change Conference in Poland.
And U.N. General Assembly President Maria Espinosa said at the rate we're seeing today, mankind is "in danger of disappearing". We need to act urgently and with audacity. "Be ambitious, but also responsible for future generations".
The demonstrators were demanding that the Belgian authorities take action to meet the commitments set in Paris in 2015 of limiting warming to "well below" a rise of 2°C above pre-industrial times while "pursuing efforts" for the tougher 1.5° goal.
Yet political and United Nations leaders have been struggling to inject urgency into two weeks of haggling on how to move on from fossil fuels to give practical effect to the Paris accord.
It was the COP21 which lead to the famed Paris agreement in 2015, the legally binding agreement to climate change where countries are obliged to keep global warming well below 2 degrees celsius.
The goal of the conference is to come up with a "rulebook" to enforce the Paris agreement, even as some recent reports claim that the agreement does not go far enough, according to AFP. Some are on track, others aren't.
Delegates from almost 200 countries now have two weeks of negotiations to finalize how those goals work in practice, even as science suggests the pace of climate change is rapidly outstripping mankind's response.
"The world is at a crossroads and decisive action in the next two years will be crucial to tackle these urgent threats".
The UN has said the main greenhouse gas emissions which warms Earth - carbon dioxide, methane and nitrous oxide - have all reached record levels.
"In short, we need a complete transformation of our global energy economy, as well as how we manage land and forest resources", Mr Guterres said.
Host-nation president Andrzej Duda said Poland, which relies on coal for about 80 per cent of the nation's energy, has no plan to give up coal entirely and that the country's use of it doesn't get in the way with fighting climate change. However, Poland did announce it will cut its reliance on coal to about 50 per cent by 2030. The country's supply of coal can last another 200 years.
But past year President Trump shocked the global community when he pulled the U.S. out of the agreement, saying he would negotiate a new "fair" deal which would not put American businesses and workers at a disadvantage.