More than 2,700 climate events in 158 countries have taken place around the world today, including rallies in New Delhi, Jakarta, London, Melbourne and Rio de Janeiro.
Rallies took place in New Delhi, London, New York and Sydney, where G20 talks are taking place.
A People's Climate Picnic, organised by a range of charity and environmental organisations, took place in St Stephen's Green in Dublin this afternoon to mark the worldwide day of action.
Celebrities, activists and political leaders joined more than 100,000 people in New York for what could be the largest climate change protest in history.
The UN's special envoy on climate change, former president Mary Robinson, says UN chief Mr Ban's attendance is to put pressure on world leaders for more action.
Mr Ban is urging governments to support an ambitious global agreement to dramatically reduce global warming pollution.
More than 120 heads of state and almost 40 ministers are due to attend the summit that Mr Ban hopes will energise negotiations on reaching a deal on cutting greenhouse gas emissions.
The meeting will set the stage for a crucial conference in Paris in December 2015 aimed at finalising an agreement.
Organisers said 550 busloads had arrived for the rally in New York, which followed similar events in 166 countries including Britain, France, Afghanistan and Bulgaria.
Organisers billed the event as the largest gathering focused on climate change since 2009.
Tens of thousands gathered in Copenhagen in a sometime raucous demonstration that resulted in the detention of 2,000 protesters.