The two-week conference, the climax of two years of contentious negotiations, convened in an upbeat mood after a series of promises by rich and emerging economies to curb their greenhouse gases, but with major issues yet to be resolved.
Conference president Connie Hedegaard said the key to an agreement is finding a way to raise and channel public and private financing to poor countries for years to come to help them fight the effects of climate change.
Hedegaard - Denmark's former climate minister - said if governments miss their chance at the Copenhagen summit, a better opportunity may never come.
"This is our chance. If we miss it, it could take years before we got a new and better one. If we ever do," she said.
Denmark's prime minister said 110 heads of state and government will attend the final days of the conference. President Barack Obama's decision to attend the end of the conference, not the middle, was taken as a signal that an agreement was getting closer.
The conference opened with video clips of children from around the globe urging delegates to help them grow up in a world without catastrophic warming. On the sidelines, climate activists competed for attention to their campaigns on deforestation, clean energy and low-carbon growth.
Mohamad Shinaz, an activist from the Maldives, plunged feet-first into a tank with nearly 200 gallons (750 liters) of frigid water to illustrate what rising sea levels were doing to his island nation.
"I want people to know that this is happening," Shinaz said, the water reaching up to his chest. "We have to stop global warming."
At stake is a deal that aims to wean the world away from fossil fuels and other pollutants to greener sources of energy, and to transfer hundreds of billions of dollars from rich to poor countries every year over decades to help them adapt to climate change.
Scientists say without such an agreement, the Earth will face the consequences of ever-rising temperatures, leading to the extinction of plant and animal species, the flooding of coastal cities, more extreme weather events, drought and the spread of diseases.
"The evidence is now overwhelming" that the world needs early action to combat global warming, said Rajendra Pachauri, the head of the Intergovernmental Panel on Climate Change, an U.N. expert panel.
He defended climate research in the face of a controversy over e-mails pilfered from a British university, which global warming skeptics say show scientists have been conspiring to hide evidence that doesn't fit their theories.
"The recent incident of stealing the e-mails of scientists at the University of East Anglia shows that some would go to the extent of carrying out illegal acts perhaps in an attempt to discredit the IPCC," he told the conference.
Negotiations have dragged on for two years, only recently showing signs of breakthroughs with new commitments from The United States, China and India to control greenhouse gas emissions.
The first week of the conference will focus on refining the complex text of a draft treaty. But major decisions will await the arrival next week of environment ministers and the heads of state in the final days of the conference, which ends Dec. 18.
"The time for formal statements is over. The time for restating well-known positions is past," said the U.N.'s top climate official, Yvo de Boer. "Copenhagen will only be a success it delivers significant and immediate action."
Among those decisions is a proposed fund of $10 billion each year for the next three years to help poor countries create climate change strategies. After that, hundreds of billions of dollars will be needed every year to set the world on a new energy path and adapt to new climates.
"The deal that we invite leaders to sign up on will be one that affects all aspects of society, just as the changing climate does," said Danish Prime Minister Lars Loekke Rasmussen. "Negotiators cannot do this alone, nor can politicians. The ultimate responsibility rests with the citizens of the world, who will ultimately bear the fatal consequences if we fail to act."
A study released by the U.N. Environment Program on Sunday indicated that pledges by industrial countries and major emerging nations fall just short of the reductions of greenhouse gas emissions that scientists have said are needed to keep average temperatures from rising more than 2 degrees C (3.6 F).
Associated Press writer Karl Ritter contributed to this report.
EDITOR'S NOTE - Find behind-the-scenes information, blog posts and discussion about the Copenhagen climate conference at http://www.facebook.com/theclimatepool, a Facebook page run by AP and an array of international news agencies. Follow coverage and blogging of the event on Twitter at: http://www.twitter.com/AP_ClimatePool.