Politicians and scientists urged the talks to agree immediate action to curb greenhouse gases and come up with billions of dollars in aid and technology to help the poor.
Representatives from 192 countries and some 15,000 delegates are attending the 7-18 December negotiations in Copenhagen aimed at tackling climate change.
Danish Prime Minister Lars Lokke Rasmussen said 110 world leaders, including US President Barack Obama, were signed up to attend a summit at the end of the December meeting.
'A deal is within our reach,' Mr Rasmussen said.
However, the talks will have to overcome deep distrust between rich and poor nations on sharing out the burden of curbing emissions, mainly from burning fossil fuels.
The presence of so many world leaders meant 'an opportunity the world cannot afford to miss,' he said of the talks, aimed at agreeing a pact to replace the existing UN Kyoto Protocol that runs to 2012.
At the opening ceremony, Mr Rasmussen said that 'the world is depositing hope with you for a short while in the history of humanity'.
He added: 'For the next two weeks, Copenhagen will be Hopenhagen.
'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 clock has ticked down to zero. After two years of negotiations the time has come to deliver,' said Yvo de Boer, the head of the UN Climate Change Secretariat.
Rajendra Pachauri, the head of the UN panel of climate scientists, said action was needed to avoid more intense cyclones, heatwaves, floods, and possible loss of the Greenland ice sheet, which could mean a sea level rise of 7 metres over centuries.
He said that even a widely accepted goal of limiting global warming to a maximum of 2C over pre-industrial times could still bring an increase in sea levels that 'could submerge several small island states and Bangladesh'.
'The evidence is now overwhelming that the world would benefit greatly from early action, and that delay would only lead to costs in economic and human terms that would become progressively high,' he said.
Newspapers call for decisive action
Newspapers in 45 countries have issued world leaders gathering in Copenhagen with a stark choice - act decisively to save humanity from a 'profound emergency' or allow climate change to 'ravage our planet'.
56 newspapers published a common editorial warning that failure to reach a 'fair and effective' deal at the crucial UN talks will spell disaster for future generations.
Most of the newspapers have taken the unusual step of featuring the editorial on their front pages, according to The Guardian which drafted the piece during more than a month of consultations with editors ahead of the summit.
It is to be published in 20 languages including Chinese, Arabic and Russian in newspapers around the world ranging from The Irish Times to France's Le Monde and the Cambodia Daily.
The editorial begins: 'Today 56 newspapers in 45 countries take the unprecedented step of speaking with one voice through a common editorial. We do so because humanity faces a profound emergency.
'Unless we combine to take decisive action, climate change will ravage our planet, and with it our prosperity and security.'
It continues: 'Climate change has been caused over centuries, has consequences that will endure for all time and our prospects of taming it will be determined in the next 14 days.
'We call on the representatives of the 192 countries gathered in Copenhagen not to hesitate, not to fall into dispute, not to blame each other but to seize opportunity from the greatest failure of modern politics.
'This should not be a fight between the rich world and the poor world, or between east and west. Climate change affects everyone, and must be solved by everyone.'
The facts behind climate change are clear, it said. The world needs to take steps to limit temperature rises to 2C, which will require global emissions to peak and begin falling within the next five to ten years.
The editorial mentions the recent row over leaked emails from the University of East Anglia's Climatic Research Unit, which climate sceptics allege show scientists manipulating data to support a theory of man-made global warming.
But while acknowledging that the controversy has 'muddied the waters', it said it ultimately failed to dent the mass of climate change evidence.
'The dangers have been becoming apparent for a generation. Now the facts have started to speak: 11 of the past 14 years have been the warmest on record, the Arctic ice-cap is melting and last year's inflamed oil and food prices provide a foretaste of future havoc.
'In scientific journals the question is no longer whether humans are to blame, but how little time we have left to limit the damage.
'Yet so far the world's response has been feeble and half-hearted.'
It concludes with a plea to politicians in Copenhagen to 'make the right choice'.
