Protesters who had gathered in small groups around the City descended on carbon trading body the European Climate Exchange on Bishopsgate, shutting off a stretch of road and pitching tents in a matter of minutes.
The climate campers are protesting against carbon trading, which they say is not an effective way to reduce greenhouse gases as it allows rich countries to carry on polluting while "offsetting" their emissions elsewhere.
They are also concerned about financial stimulus measures by the G20 nations to combat recession, because it will restore a growth economy which causes increased emissions.
On the eve of the G20 summit, environmental groups including Greenpeace, Friends of the Earth and WWF warned that the leaders of the world's major economies risked missing a unique opportunity to tackle the recession and climate change simultaneously.
The environmental groups are calling for leaders to avoid a return to "business as usual", with investment in green measures such as low-carbon technology to boost jobs and slash the emissions which cause climate change.
In contrast to G20 protests elsewhere in the City, today's climate camp was peaceful throughout the afternoon, with police looking on as demonstrators took part in workshops, a farmers' market stall, music, giant boardgames and even meditation.
The climate campers, who decorated the street and their tents with banners, bunting and flowers, aim to stay in the Bishopsgate site for 24 hours, with bicycle-powered cinema and a ceilidh tonight.
Protester Richard Howlett said politicians had responded to previous camps at Heathrow Airport and Kingsnorth power station in Kent by claiming carbon trading would solve the problem of increased emissions from a new runway or coal-fired plant.
"Anybody with any common sense can see that we're not going to get ourselves on the low-carbon economy we need if we're building new runways and new coal-fired power stations," he said.
"Carbon trading has been the false solution that has been thrown at us when we've held previous camps and we want to tackle that head-on today.
"The other point we want to make is that the G20 is meeting up to shore up an economic system that has been disastrous in many ways; one way it guarantees to create disaster is the fact that it's got us on course for runaway climate change."
Another member of the climate camp, Kevin Smith, said the market-based approach of carbon trading to solving climate change had proven to be "spectacularly unstable and ineffective".
"We have such a limited window of opportunity to act on climate change; it seems really foolish that we're trying to do this big experiment because it fits with the free market ideology that's been so fashionable for the last decade," he said.
"The current economic model is based on infinite growth on an infinite planet, and to anyone with common sense that does not work."
Other climate protesters staged a demonstration near to the ExCel centre in London's Docklands, where tomorrow's G20 summit is being held.
The Campaign against Climate Change chose an iceberg sculpture to highlight the melting of Arctic sea ice, and called for global warming to be a top priority for politicians.
The group's national co-ordinator Phil Thornhill said: "Some of them want to put a lot of money into the global economy.
"The way to do that is to spend on the things we need to do to fight climate change and reduce emissions.
"There's loads of things that need doing, massive infrastructure projects, new grids, renewable energy, marine wind farms, tidal power which has huge potential, all of this could be green jobs.
"This is doing two things at once, it wouldn't just be putting the money into a black hole which they are now."