The biggest climate meeting in history has opened in Copenhagen with hosts Denmark saying an unmissable opportunity to protect the planet was "within reach".
"The world is depositing hope with you for a short while in the history of mankind," Lars Lokke Rasmussen, the Danish prime minister told delegates who are seeking to agree the first United Nations climate pact in 12 years.
Rasmussen said: "By the end, we must be able to deliver back to the world what was granted us here today: hope for a better future.
"The time for formal statements is over ... Copenhagen will only be a success if it delivers significant and immediate action."
The high-stakes talks are aimed at agreeing a deal for measures to help curb greenhouse gas emissions and the transfer of hundreds of billions of dollars from rich to poor countries to help developing nations adapt to climate change over the coming years.
A major challenge facing the two-week conference, which ends with a meeting of 105 world leaders on December 18, is to overcome deep distrust between rich and poor nations about sharing out the burden of costly curbs on emissions.
A top UN official warned on Monday that providing the money to poorer countries to tackle climate change must not take away urgently needed resources from other issues facing developing countries, such as poverty and hunger.
"Before we put all our efforts into climate change, poor countries were faced with a list of other problems," said Kandeh Yumkella, director general of the United Nations Industrial Development Organisation (Unido).
Asked what position Unido would take at the climate talks, Yumkella said: "We don't want climate change to cannibalise other development financing.
"To deal with the climate change problem, developing countries are looking for fresh money."
He called on industrialised countries to change their consumption and production habits "to send a signal that indeed we do care about the others".
"The request for putting money on the table is not because developing countries just want cash," the Unido chief said.
"The fact is there's a moral and equity question here: those who polluted least are suffering the most. There's a moral requirement here that we have to step up to.
"We cannot have two worlds in the 21st century where we say to some: slow your growth, keep the forests alive so we can do what we were doing before."
Earlier, Yvo de Boer, the head of the UN framework convention on climate change, said he was confident the summit would be a success.
"Copenhagen is already a turning point in the international response to climate change," he said.
De Boer said developing nations need to take new action to slow the rise of their emissions.
He said he also wanted rich nations to accept deep cuts to their emissions by 2020 and come up with at least $10bn a year in aid to the poor countries to kick off a deal, saying it has to be "new money, real and significant".
The planned attendance of heads of state and pledges to curb emissions by all the top emitters – led by China, the US, Russia and India – have raised hopes for an accord after sluggish negotiations in the past two years.
Manmohan Singh, the Indian prime minister whose country is the world's fourth-largest greenhouse gas emitter, announced on Saturday that he would attend the closing summit, joining 104 other leaders, including Barack Obama, the US president.
Meanwhile, the head of the UN's panel of climate scientists strongly defended findings that humans are warming the planet, after critics said that leaked emails from a British university had undermined evidence.
| Pachauri strongly defended findings that humans are warming the planet [EPA]
Climate change sceptics have seized on a series of hacked emails written by climate specialists, accusing them of colluding to suppress others' data and enhance their own.
Speaking on Monday, Rajendra Pachauri, head of the UN's Intergovernmental Panel on Climate Change (IPCC), said: "The evidence is now overwhelming that the world would benefit greatly from early action.
"The recent incident of stealing the emails of scientists at the University of East Anglia shows that some [people] would go to the extent of carrying out illegal acts perhaps in an attempt to discredit the IPCC."
In 2007, the UN panel said that it was at least 90 per cent certain that humans were to blame for global warming.
The climate talks have sparked protests in many European cities, adding to the pressure world leaders are under to reduce rising emissions that the UN says will cause desertification, mudslides, more powerful cyclones, rising sea levels and the extinction of species.
The existing Kyoto pact obliges industrialised nations to cut emissions until 2012, and the idea behind the Copenhagen talks is to get action from all major emitters, including China and India, which were exempt from the Kyoto agreement.
The meeting will test how far developing nations will stick to entrenched positions, for example that rich nations must cut their greenhouse gases by at least 40 per cent by 2020 – far deeper than targets on offer.
World leaders did not attend the last time the world's environment ministers agreed on the existing UN climate pact, known as the Kyoto Protocol, in 1997.
Copenhagen is the biggest climate change meeting in history, with 15,000 participants from more than 190 nations.