United Nations Secretary-General Ban Ki-moon urged world environment ministers to compromise in these final days of talks at the climate change conference in Copenhagen, as worries grew that the talks would end without a substantive treaty.
"Three years of effort have come down to three days of action," Ban said. "Let us not falter in the home stretch. No one will get everything they want in this negotiation."
The talks have become bogged down by disputes about how much rich countries are expected to cut greenhouse gas emissions and what actions poorer nations should take to fight climate change.
Global giants China and the United States have been locking horns over who who should pay for emission cuts with, China -- considered a developing nation at the talks -- says it needs aid from developed nations.
The world's largest polluter is also resisting attempts from the U.S. and other nations to make their cuts binding and open to international scrutiny, rather than voluntary.
Conference president, Denmark's Connie Hedegaard, said she hoped the looming Friday deadline would pressure leaders to make compromises and break the deadlock.
"It's just like schoolchildren. If they have a very long deadline to deliver an exercise they will wait for the last moment... it's basically as simple as that," she told Reuters.
It's a bad omen for a week that started with developing countries temporarily boycotting the Copenhagen negotiations. The countries refused to participate further saying they feared industrial countries were backpedalling in their promises to cut greenhouse gases.
After an hours-long standoff, the negotiations resumed later in the evening when developing nations were assured their objections would be heard. But deep divisions remain.
With time running out on the two-week-long conference, the chances that a legally binding treaty will emerge have dimmed, with the focus now turning to the hope that a substantive political pact can be reached instead.
"The next few days are crunch days," The Globe and Mail's Eric Reguly told CTV's Canada AM from Copenhagen. "But I suspect that with all these world leaders arriving, they've got to pull something out of a hat, and fast."
More than 110 world leaders have begun to arrive for the conference. British Prime Minister Gordon Brown was planning to arrive Tuesday -- a day earlier than planned -- to help push the talks forward. U.S. President Barack Obama, Prime Minister Stephen Harper and Chinese Prime Minister Wen Jiabao are expected later this week.
Reguly says with more than two years of preparation and talks, leaders are under pressure to make a deal during this high-level stage of the conference.
"But it might be a deal so weak that it's merely a licence to keep these talks going somewhere else sometime next year," he noted.
The largest stumbling block at the talks concerns the future of the Kyoto Protocol, which compels only 40 industrialized nations to limit greenhouse gas emissions.
Developing nations want to extend the 1997 Protocol -- which binds the nations to cut greenhouse gas emissions below 1990 levels until 2012 -- and then work out a separate new deal for poor nations.
But most rich nations want to merge Kyoto into a single new accord obliging all nations to fight global warming.
Hedegaard said environment ministers already in Copenhagen worked late into the night Monday to resolve outstanding issues.
"Ministers have to be very clear and focused over the next 48 hours if we are to make it," she said.
Canada's environment minister Jim Prentice said he is still hopeful leaders can reach an agreement in principle by Friday.
"That will make up the base and schedule from which will move forward in 2010 to arrive at an international treaty," he said. "That is our objective and we hope that our leaders will reach it on Thursday and Friday."
London protesters deface Canadian flag
Meanwhile, British police arrested three environmental activists who defaced and cut down Canada's flag at the Canadian High Commission in London.
The three men, all in their 20s, scaled the building's wall around 8 a.m. Tuesday, painted the flag and then cut it down to protest Canada's oilsands industry.
The men also chained themselves to a balcony, but surrendered peacefully after about an hour.
The protesters are members of the Camp for Climate Action group, which accuses Harper of obstructing progress at the Copenhagen summit to protect the oilsands industry.
"Tarsands are the dirtiest fuel known to man, both in terms of their impact on the climate and the devastation inflicted on the local communities," the group said in a statement.
The protesters are charged with aggravated trespassing and criminal damage.
With reports from The Associated Press
