Final preparations were underway here Sunday for a marathon UN conference on climate change, amid beefed-up security, activists' clamour and warnings that a hothouse planet loomed if the talks failed. UN Secretary General Ban Ki-moon issued an upbeat note on the eve of the landmark conference, predicting mankind would strike a blow against the 21st century's great peril. The climax on December 18 -- to be attended by more than 100 heads of state and government -- should deliver a framework for a historic pact, he said. 'I am convinced that the conference in Copenhagen will give us a strong and important political accord which will then be the basis for an accord that is legally binding,' Ban told French television. The December 7-18 conference gathers 192 nations under the flag of the UN Framework Convention on Climate Change (UNFCCC), the offshoot of the famous 1992 Rio summit. Danish police on Sunday tightened security around the conference venue, the Bella Center, closing off access for about an hour to investigate a bomb alert. Six thousand police -- more than half of all the police in Denmark -- are being deployed in the capital. They could be reinforced to 9,300 if need be, according to Mogens Lauridsen, head of operations at Copenhagen police. Greenpeace's flagship, Arctic Sunrise, anchored in the harbour in central Copenhagen, unfurling a banner reading: 'OUR CLIMATE -- OUR FUTURE -- YOUR DECISION.' Nearby, a self-styled Alternative Climate Forum geared up to accommodate some 10,000 visitors per day, with 6,000 already on site. In London, meanwhile, more than 100 environmental campaigners camped out overnight in Trafalgar Square, organisers said Sunday, adding that they intend to stay till the UN climate summit opened. Two years of talks have led up to Copenhagen. The goal is to deliver an accord that will ratchet up efforts against climate change, driven by uncontrolled emissions of heat-trapping carbon gases from fossil fuels. An outline accord in Copenhagen would be fleshed out in negotiations next year and take effect from 2013, when current pledges expire under the UNFCCC's Kyoto Protocol expire. 'Time is up,' UNFCCC chief Yvo de Boer told a press conference. 'Over the next two weeks, governments have to deliver a strong and long-term response to climate change.' Analysts, though, see many pitfalls in the coming days, as rich and poor countries -- and advanced economies among themselves -- wrangle over burden- sharing. De Boer threw his weight behind a fast-track funding proposal, endorsed by the European Union (EU), that would disburse 30 billion dollars in aid to poor countries over the next three years before the new treaty takes effect. The money would be used to help these countries ease carbon emissions and shore up defences against climate change. 'Clearly, though, over time, by 2020 or 2030, we are going to need more significant sums, in the hundreds of billions of dollars, to deal with both [emissions] mitigation and [climate] adaption,' he said. Meanwhile, scientists in Germany said current pledges for reducing carbon pollution would doom Earth to potentially catastrophic warming by century's end. National commitments proposed so far would mean the global temperature would rise by 3.5 degrees Celsius (6.3 degrees Fahrenheit) over pre-industrial times, way over a 2.0 C (3.6 F) threshold widely considered safe, according to the tally published by the Potsdam Institute for Climate Impact Research and energy specialists Ecofys. 'Global emissions are likely to be nearly double 1990 levels by 2040 based on present pledges,' it said. In contrast, a report by climate economist Nicholas Stern and the UN Environment Programme (UNEP) concluded that closing the 'emissions gap' could be somewhat easier than thought. 'Existing proposals from developed and developing countries constitute a big step towards a level consistent with the 2 C (3.6 F) goal,' which would require that CO2 emissions stay under 44 billion tonnes in 2020, their report said. 'Taking countries' highest intentions would take the world to around 46 billion tonnes (of carbon),' meaning that only a two-billion-tonne shortfall would have to be bridged. 