Speakers set to address the summit on Thursday include Ms Merkel, Mr Brown, Brazilian President Luiz Inacio Lula de Silva and Iranian President Mahmoud Ahmadinejad.
US President Barack Obama is due to attend the final day of the meeting on Friday, when world leaders will try to lay out a strategy to deal with climate change after the end of 2012, when obligations run out under the landmark Kyoto Protocol.
Developing countries, led by China, accused host Denmark of a lack of transparency by suggesting language for the agreement without full consultation by all sides on the 194-nation summit.
And China told participants that it saw no chance of reaching an operational accord this week, an unnamed official told Reuters news agency.
Danish Prime Minister Lars Loekke Rasmussen has taken charge of the Wednesday-Friday segment involving heads of state and government.
Denmark said it was trying to simplify several complex draft negotiating texts to help the world leaders to agree on a deal.
But BBC environment correspondent Roger Harrabin, in Copenhagen, says the Danish hosts do not appear to have worked out how the political leaders will take part in the formal UN negotiating process.
That process itself has been in disarray, our correspondent says, with nations refusing to agree with the Danish prime minister's demands that they should negotiate on a slimmed down text.
Containing emissions to a level associated with a temperature rise of no more than 2C is the stated aim of the big nations here.
As things are going they will miss that target by a considerable margin, our correspondent says.
The poorest and most vulnerable nations say emissions should be contained to a level associated with a temperature rise of 1 or 1.5C.
They have no chance of getting their way, our correspondent adds.
After a day of long delays and finger-pointing on Wednesday, Britain's Energy and Climate Change Secretary, Ed Milliband, said the prospects for reaching a deal were on a knife-edge.
"People can kill this process, kill the agreement with process argument," Mr Miliband said, warning the talks were at a "very dangerous point".
But BBC environment correspondent Richard Black says that for developing countries, the ongoing wrangles about how negotiations should proceed is not a matter of process but of substance.
They say they have been negotiating on and off for nine months, and want to see that process through to its conclusion.
The president of the European Commission, Jose Manuel Barroso, also said he was disappointed with the slow pace of negotiations, whilst Venezuelan President Hugo Chavez said: "If the climate was a bank, a capitalist bank, [the West] would have saved it by now."
On a more promising note, Japan promised poorer nations $15bn (£9bn) over three years if a deal is made.
The amount - payable from 2010-2012 - adds to the $10.6bn (£6.5bn) commitment over three years made by EU leaders at their summit last week.
Another pledge came from a six-member group - Australia, France, Japan, Norway, the UK and US - which will collectively commit $3.5bn over three years to combat deforestation.
Ethiopian Prime Minister Meles Zenawi, speaking on behalf of the African Union, also announced dramatically reduced expectations for climate aid from rich nations.
Tensions also flared outside the summit, where police used clubs and tear gas to stop some 2,500 activists who tried to march on the tightly-guarded Bella Center.
Police detained some 260 protesters during the clashes, which led to injuries on both sides.
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