Holding placards with slogans such as "cut carbon pollution, unlock clean energy" and "say yes to cutting carbon pollution", they gathered at Sydney's Prince Alfred Park on Sunday morning to deliver a message: climate change is happening, and something needs to be done.
"What people are asking for is an ambitious price (on carbon), an investment in renewable energy," rally organiser and national director of GetUp, Simon Sheik, said.
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"Today is a big day, because today Australians will ask their government for a price on carbon."
Simultaneous rallies were being held in most capital cities as the second stage of the "Say Yes" campaign launched late last week by actors Cate Blanchett and Michael Caton.
The advertisement, which urges Australians to say yes to the federal government's proposed carbon emissions tax, stirred controversy among some sections of the media.
Community climate advocate Ramya Krishnan slammed the controversy surrounding Ms Blanchett's contribution to the campaign.
"I hear about families who are struggling just like everyone else who want to live in a better world for their children to grow up.
"The shock jocks don't speak for Western Sydney, and neither does Tony Abbott."
Police said the Sydney crowd numbered between 7500 and 8000.
As part of the campaign, rally attendees were handed unsigned letters to hand out to their neighbours, arguing the case for climate action.
The letter attacks the claim the carbon tax will be directly levied against Australian families.
Instead, it argues the tax will affect fewer than 1000 of the biggest polluting companies in Australia and that they will have to pay a "small price" per tonne of carbon pollution.
The opposition has repeatedly attacked the proposed tax on carbon as damaging to Australian industry and instead favours a "direct action", small target approach to climate change.
Prime Minister Julia Gillard's popularity has taken a beating in recent months since she announced her government would pursue a carbon tax, without announcing detail on the amount of tax.
