Tim Friend, Al Jazeera's correspondent in London, said: "There's a carnival atmosphere here ... What they've succeeded in doing is encircling the Palace of Westminster and performed this great wave.
"They say they want the Copenhagen summit to commit to reducing carbon emissions by 40 per cent over 10 years.
"No one has come close to making that kind of commitment, so although we've had an optimistic atmosphere here today, I suspect at the end of the summit, some of these people are going to be disappointed."
Barbara Stocking, Oxfam Great Britain's chief executive, said: "The UK government must fight for a comprehensive, fair and binding deal at Copenhagen - that is our demand today and we expect it to be fulfilled."
"They must return home with a strong, effective climate deal both for our own sakes in the UK and for the millions of poor people already suffering from the effects of climate change around the world."
The UK's Met Office said on Saturday it would publish some of the data it uses to analyse climate change, after thousands of pieces of correspondence between some of the world's leading climate scientists were stolen from the UK's University of East Anglia and leaked to the internet.
Sceptics of global warming as a man-made phenomenon have said the data proves that scientists have been conspiring to hide evidence about climate change.
On Friday, Rajendra Pachauri, the chairman of the Intergovernmental Panel on Climate Change (IPCC), said the issue raised by the emails was serious and would be looked at in detail.
Barry Gromett, a Met Office spokesman, said data from 1,000 weather stations around the world, covering 150 years, will be released early next week.
The office has written to 188 countries to ask for permission to release more data from a further 4,000 stations.
Ahead of the march through central London, Rowan Williams, the Archbishop of Canterbury, offered prayers for negotiators in Copenhagen and urged people not to listen to those who say there is a choice between "looking after human beings and looking after the Earth".
Around 15,000 delegates, environmentalists, scientists, journalists and others are due to gather in Copenhagen on Monday to begin two weeks of negotiations on what to do about climate change.