NSW Police Inspector Gary Coffey said "it’s a very big crowd", and later told the Herald there were an estimated 20,000 people in attendance.
Protesters march in Sydney to demand climate change action
Chloe Rafferty, one of the organisers, said she was angry about the lack of climate action from all levels of government.
"The state is angry, Sydney is angry," she said. "I have hope that people will see the need to take action into their own hands and disrupt business as usual, we can't let the biggest city in Australia having hazardous air quality become the new normal."
High school student Amy Lamont addressed the thousands of protesters wearing P2 face masks and said: "The reality is these fires will be around all summer."
"The rage we all rightly feel right now needs to grow if we have any chance of actually challenging that destruction of the status quo that is burning around us," she said.
"Only we, the majority, have the ability to hold the rich and political elites in this country accountable.......