'We have the power to change the world': Thousands march in cities from New Delhi to New York demanding action on climate change
40,000 campaigners in London
2,000 events taking place in 150 countries ahead of United Nations climate summit next week
100,000 people expected to get involved in demonstration in summit's location of New York
Thousands take part in marches in Sydney and Melbourne, and in India, Turkey and Belgium
|30,000 in Melbourne|
The marches on Sunday were part of a series of events across the world that will culminate in what is expected to be a massive march in New York City where the United Nations climate summit is being held.
United Nations secretary-general Ban Ki-moon, who is hosting the summit, is expected to join the New York rally.
"Climate change is the defining issue of our time. The longer we delay our action, the more we will have to pay," he said.
"That is why I need everybody on deck - governments, business communities and civil societies."
About 2,000 marches were expected to take place in the lead up to the UN summit, which is to be attended by more than 100 world leaders. Prime Minister Tony Abbott is not attending the event.
Locally, more than 30,000 people took to the streets in Melbourne and about 1,500 in Brisbane, with many more attending events in other capital cities.
Greens leader Christine Milne delivered a speech at the event in Melbourne, urging protesters to think about those living in climate change-affected places like Kiribati and Tuvalu in the Pacific.
"They are living the impacts of climate change right now and that's why the absolute message for today is actions not words," she said.
"We don't just need to start a conversation about what we need to do, we need to take action, that's what we need to do."
A focus on increasing renewable energy in Australia and setting climate change targets were some of the key issues raised by protesters.
Demonstrators carried banners reading: "Tony Abbott: Climate Change is on my agenda."
Labor's environment spokesman Mark Butler said the Prime Minister should take note of the message and include climate change issues on the agenda at the G20 finance meeting that is underway in Cairns.
"All responsible players around the world have indicated as much this is an economic, a social and an environmental challenge," he said.
"A very serious series of meetings without addressing climate change is simply missing out on that opportunity and that challenge."