About 2500 similar rallies were held in 162 countries, the largest of which was a protest in Manhattan, New York, where world leaders will gather to discuss climate change at a United Nations summit on Tuesday.

The display of global unity has been dubbed The People's Climate Mobilisation.

In Sydney, about 1000 protesters formed a human sign saying "beyond coal and gas". In Melbourne, a crowd of 30,000 marched through the streets campaigning for greater use of renewable energy, particularly solar and wind.

The summit was called by the United Nations secretary general, Ban Ki-moon, as a prelude to 15 months of negotiations in a new international climate agreement expected to be ratified late next year.

Foreign Affairs Minister Julie Bishop will represent Australia at the summit while Prime Minister Tony Abbott will head to the United States the following day for a UN Security Council meeting on terrorism.

Scientists believe climate change will cause serious problems if emissions continue at the current pace.

Climate change is caused by trapping excess carbon in the Earth's atmosphere. This trapped pollution heats up, altering the Earth's climate patterns. The largest source of pollution is the burning of fossil fuels for energy.

Tom Hunt, organiser of the Wollongong Climate Action Network, told protesters that the atmosphere already had 40 per cent more carbon dioxide than it did 200 years ago.

"The science is clear. We know climate change is happening and that it's serious," he said. "We can change things but we can't seem to stop ourselves. Our society has grown far too reliant on fossil fuels."

Mr Hunt called on the Abbott Government to stop growth in Australia's coal and gas mining.