Thousands rallied in Sydney on Wednesday evening to demand urgent climate action from Australia's government, as bushfire smoke chokes the city and causes a spike in health problems.

A mass of people crowded Town Hall, with one police officer at the scene estimating the number of protesters may have exceeded 7,500.

AAP

Some wore the face masks people have donned to cope with the smoke haze, while others brandished signs reading: "Denial is not a policy", "Less debate more climate action", "For my grandkids" and "Climate change is a public health emergency".

Smoke alarms rang out across Australia's biggest city on Tuesday, causing the evacuation of workplaces, train stations and even the NSW Rural Fire Service headquarters, thanks to a thick haze from bushfires burning along the country's east coast.

sam langfor

Sydney has endured weeks bathed in toxic smoke as hundreds of blazes have raged across the countryside, with hospitals recording a 25 per cent increase in the number of people visiting emergency departments last week.

Walter and Marie, both in their 70s, say they are “increasingly concerned about the climate crisis”. Marie lives in the Blue Mountains, where “smoke and fire are invading from all directions.” @sbsnews #nswpol #auspol #AustraliaFires pic.twitter.com/M1HzhJzajy — Evan Young (@evanb_y) December 11, 2019

The devastating fires have focused attention on climate change, with scientists saying the blazes have come earlier and with more intensity than usual due to global warming and a prolonged drought.

Evan Young

Among the thousands gathered in the city, one protester told SBS News she was "pissed off".

"I'm here for the same reason everyone else is," Claire, 60, said.

Walter and Marie, both in their 70s, said they were attending the rally out of fear of what would happen next, saying they are “increasingly concerned about the climate crisis”.

Supplied / Sally Murrell

Marie lives in the Blue Mountains, west of Sydney, and has already seen bushfire damage first hand.

“Smoke and fire are invading from all directions,” she said.

"The country is on fire," said 26-year-old Samuel Wilkie, attending his first climate protest. He described politicians' response as "pathetic".

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Protester Elise Vohradsky, from Western Sydney, said she was motivated to attend after seeing smoke shroud the city on Tuesday.

She said it was upsetting to see Prime Minister Scott Morrison speaking in Sydney about the government's religious freedom bill that same day.

"I went outside and couldn't even see anything, like it was just smog. I had to like put a scarf around my mouth and nose to just go to the shops across the road," she told AAP.

"There's so much devastation - and not just the people, the cities but the environment, the animals.

"I'm pretty angry about it all. I want to do something and I don't know what I can do."

"Our government is not doing anything about it," said 29-year-old landscape gardener Zara Zoe.

"No one is listening, no one is doing anything."

Sydney man Patrick Cullen attended the rally wearing a filter mask.

Patrick Cullen

"I never thought I'd have to rally for my right to breathe," he told SBS News.

"Our state is on fire, our firies are massively underfunded and we are 11 days into summer."

Organiser Chloe Rafferty said thousands had rallied because bushfires and smoke haze had created anger at the conservative government's inaction on curbing emissions.

READ MORE Sydney workers evacuated as smoke triggers alarms at offices, train stations

"I think the wider public can see that we are not expecting the climate crisis in the future but we are facing the climate crisis now," she said.

"People are experiencing it in their day-to-day lives."

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As well as a rise in people visiting hospitals with smoke-related health symptoms, the number of emergency calls for ambulances spiked 30 per cent last week.

"For most people, smoke causes mild symptoms like sore eyes, nose and throat," top health department official Richard Broome said in a statement.

"However, people with conditions like asthma, emphysema and angina are at greater risk because the smoke can trigger their symptoms."

Impressive crowd at Sydney Town Hall for the #ClimateEmergency rally! pic.twitter.com/pCeqBJYicc — Chanchanok Chaichim (@cocculin) December 11, 2019

Smoke from bushfires is one of the biggest contributors to air pollution in Australia, releasing fine particles that can lodge deep within people's lungs and cause "severe" health impacts over time, according to scientist Mick Meyer from government-funded scientific research agency CSIRO.

"The impact of smoke on people remote from the fires may, on occasion, substantially exceed the direct injury to people within the fire zone," he wrote in The Conversation.

"But we currently lack the operational tools to understand the extent of these impacts or to manage them."

Six people have been killed and more than 700 houses destroyed in bushfires this fire season.

Nick, 29, is a horticulturist. “I brought this plant to be carbon positive. We are so choked up at the moment.” @sbsnews #nswpol #auspol #AustraliaFires pic.twitter.com/gyL4VTykfi — Evan Young (@evanb_y) December 11, 2019

Supplied / Shannon Longville

Three million hectares of land has been burnt - the size of some small countries - and vast swathes of koala habitat scorched.

Official data shows 2019 is on track to be one of the hottest and driest years on record in Australia.