Maldives President Mohamed Nasheed urged leaders not to repeat the 'charade' of promising action but not following through. Yet he issued a downcast condemnation: 'We know you are not listening.'
Obama, who has made tackling climate change a priority since coming into office, acknowledged that the US was slow to respond to the threat and declared: 'This is a new day.'
'We understand the gravity of the climate threat. We are determined to act,' Obama said, noting efforts by his government to reduce vehicle emissions and promote renewable energy.
But the Obama administration could be hamstrung at the all-important Copenhagen summit, from December 7-18, as major legislation to curb US emissions has stalled in the Senate, despite his own centre-left party's large majority in the upper chamber.
Obama called on emerging economies such as China and India to make the commitments necessary to lower their own growing emissions.
'We cannot meet this challenge unless all the largest emitters of greenhouse-gas pollution act together. There is no other way,' Obama said Tuesday.
President Hu said that China would work to slow the growth of its emissions by a 'notable margin' by 2020. He tasked richer nations with helping the poor adapt without destroying their economies.
'China stands ready to join hands with all countries to build an even better future for the generations to come,' said Hu, the first Chinese leader in 30 years to join the annual opening of the UN General Assembly. 'We should combine our efforts to address climate change with those to promote the growth of developing countries.'
The US and Chinese comments in part highlighted the ongoing rift between richer and poorer countries. How much wealthy economies should give developing nations to lower their own emissions has been a key stumbling block to a new global climate treaty.
Ban, who has made tackling climate change his top priority, is hoping Tuesday's gathering will inject the political will needed to complete the talks in Copenhagen for a successor to the Kyoto Protocol, the world's first climate treaty, which expires in 2012.
The climate summit comes ahead of Wednesday's annual opening of the UN General Assembly, which brings together leaders from around the world for five days of speeches about global issues.