Conference negotiators, meanwhile, went behind closed doors in talks to pin down an elusive new pact on climate, talks in which the figure 350 looms as a goal for true believers, but one that appears impossible based on progress so far.
It refers to 350 parts per million of carbon dioxide in the atmosphere, the highest concentration that some leading scientists say the world can handle without sparking dangerous climate effects.
"It's the most important number in the world," said Bill McKibben, founder of the environmental activist group 350.org. "It's the line between habitability on this planet and a really, really desolate future."
Not everyone buys into that. But an entire environmental group has sprung up around the number, pushing 350 as a goal, sporting it on T-shirts and flags waved by throngs of protesters that marched to the conference center over the weekend. About 100 nations at the U.N. climate summit have signed on to the idea of heading for 350.
Actually, the world has lived with more than 350 for a while.
The last time the Earth had 350 ppm of carbon dioxide in the air was a generation ago, in the fall of 1989. This year CO2 pushed over the 390 level. When scientists started measuring carbon dioxide in 1958 it was 315.
Since the atmosphere passed the 350 level, ice sheets have been melting and other dramatic changes have been happening. Prominent scientists notably NASA's James Hansen, one of the earliest to warn about global warming, and Rajendra Pachauri, head of the Intergovernmental Panel on Climate Change have said 350 is the only safe level of carbon dioxide in the air.
Still, many economists, political leaders, and even some scientists believe that the worst effects of global warming can be avoided even with less stringent actions.
But there is general agreement among negotiators and climate scientists that continued global warming will lead to dramatic changes that mean more widespread drought in some regions, greater flooding along coastlines, stronger storms and the loss of species.
On Sunday, hundreds of churches around the world had signed up to ring bells at 3:50 p.m. in their respective time zones.
"It was an incredibly powerful moment and to know that there are bells ringing all over Europe, up to Greenland, down into the south Pacific and every corner of the planet," McKibben said moments after the bells stopped ringing in Copenhagen.
As they tolled, more than 40 government environment chiefs and other high-level negotiators were meeting at the Danish Foreign Ministry. They were trying to bridge the gap between their positions in informal talks before the second and last week of negotiations gets under way. The week will end with the arrival of President Barack Obama and more than 100 other national leaders for the final hours of negotiation.
Sharp divisions remain between rich and poor countries on greenhouse emissions cuts and financing for developing nations to deal with climate change and shift to cleaner energy.
"I think there was recognition around the table of the urgency of what we need to achieve in the coming days," Britain's climate change secretary Ed Miliband said after Sunday's talks. "I think there needs to be more movement from everyone, more imagination, and I think we will all be striving for that."
Australia's Climate Change Minister Penny Wong also said a lot of work remains to be done.
"It's going to be tough to get an agreement by Friday but that's what we have to do," she told a news conference.
According to participants, the closed-door consultations focused on about a half-dozen plans on financing for poor countries to deal with climate change. One joint proposal by Mexico and Norway calls for a "Green Fund" for climate financing, starting with $10 billion a year in 2013, and increasing to $30 billion to $40 billion a year by 2020
Separately, a proposal aimed at saving the world's tropical forests suffered a setback Sunday, when negotiators ditched plans for faster action on the problem because of concerns that rich countries aren't willing to finance the plan. A deal on deforestation a sizable global warming factor is considered a key component of the larger pact.
For a second day in a row, police cracked down on climate activists marching through the Danish capital. More than 200 were detained as police stopped an unauthorized demonstration headed toward the city's harbor and carried out a security check of some of the participants.
Meanwhile, nearly all of the 1,000 detained on Saturday from Europe, Africa, Asia, and the U.S were released without charges. Thirteen of them were arraigned in court and faced preliminary charges of assaulting police or were let off with a warning for wearing masks, which are outlawed during demonstrations in Denmark, or carrying box-cutters or other sharp objects.
Reducing carbon dioxide levels to 350 would mean reversing the trend of the past couple of centuries. Carbon dioxide stays in the atmosphere for as long as 100 years. And the emissions cuts currently being pledged by developed countries, including the United States and European nations, are aimed at having CO2 levels peak at around 450, not 350, in coming decades.
And even that may not be possible. Some economists say the world should plan to stop at 550.
Economist Henry Jacoby, co-director of MIT's Joint Program on the Science and Policy of Global Change, has said that even 450 is "totally impossible, there's no way we can do that."
To get down to 350, civilization has to remove massive amounts of carbon dioxide from the skies, something talked about but not yet achieved. Trees and oceans suck CO2 from the atmosphere, but that process is overwhelmed by emissions from burning coal and oil. McKibben said it would probably take 40 years to get down to 350 even if emissions stopped today.
"It may be on the edge of impossible," he said Sunday. "We could do it. At the moment, there's no sign that we are going to do it."
MIT management professor John Sterman said scientifically 350 makes sense, even if economically it seem unreachable.
"We ought to have a goal of 350 and realize we're already above that," Sterman said.