Hundreds of people joined the "Climate Camp" demonstration, which saw tents, market stalls, banners and bunting set up outside the carbon-trading body, European Climate Exchange, in the City.
The environmental campaigners argue that carbon-trading is inadequate against the problems of climate change, with rich countries still polluting and offsetting their emissions.
Implementation of financial stimulus proposals by G20 nations would boost economies, leading to increased emissions, campaigners fear. They want leaders to avoid a return to "business as usual".
Unlike G20 demonstrations elsewhere in the City, the Climate Camp gathering remained peaceful throughout the day.
The campers hoped to remain overnight on the street in Bishopsgate, where bicycle-powered cinema and a ceilidh were due to provide entertainment into the evening.