The global day of action, inspired by Swedish teen Greta Thunberg, saw millions take to the streets worldwide, just days out from a major UN climate summit.

Some parts of Civic came to a standstill as protesters sang and held aloft signs bearing slogans such as 'Make Earth Cool Again' and 'Climate Now, Homework Later'. Many of them were "repurposed" from old campaign signs abandoned after this year's federal election - and the irony was not lost on students.

Speaking to a cheering crowd at Glebe Park, twelve-year-olds Arianne and Alison threw down a gauntlet to the federal government to act now in what scientists are calling the critical decade.

"We are the last generation that can save the planet," students declared, echoing expert calls to unchain economies from fossil fuels within the next few years or face catastrophic global warming.

Student organisers took to the streets with three clear demands: an end to new fossil fuel projects like the Adani coal mine threatening the Great Barrier Reef up north, a target of 100 per cent renewable energy by 2030 and funding for a fair transition to new jobs for fossil fuel workers.

While the crowd was young, there were people from all ages, trades and walks of life milling about the food trucks and stalls, as hundreds of businesses gave workers the day off. Families carried huge banners between them and a group of local beekeepers even came out in force - and veil.

Patrick Anderson, whose family invented the Flow Hive designed to make beekeeping more sustainable, said he was striking for some of the smallest victims of global warming.

"The school students and the bees," he laughed. "So many bees are endangered now and climate change [and pollution] is a big part of the reason why."

Organisers estimate a record 300,000 people joined strikes in 100 cities and towns across Australia on Friday - and about 15,000 were counted in Canberra through the gates of Glebe Park - far outstripping earlier strikes.

Thunberg, who sailed to New York for the climate summit in a zero-carbon yacht, posted on social media that Australia's numbers had set the standard for the day.

Keeping on message, students in Canberra reminded crowds to clean up after their mess at the park. "Help us clean up the mess you've made of our future too," one student said.

On stage, voices wobbled as kids spoke of a rapidly worsening future, of extinction and disease. But there was also laughter and music beneath the noise, primary schoolers playing violin, marchers belting out Johnny Farnham classics.

Collapsing on the grass after months of hard work wrangling permits and insurance, teen organisers laughed at suggestions they had been hijacked or "brainwashed" by left-wing lobby groups.

While organisations like Greenpeace and the youth-led Australian Youth Climate Coalition had lent a hand and offered advice, students had been busy for months building the movement after school.

Amelia Hollo, 13, got involved early on after researching global warming.

"When I first found out, I was really scared," she said. "I'd cry myself to sleep but being part of this has been really good, I've learnt a lot."

She said it was heartening to see so many adults joining the strike this time around.

"These so many teachers and grandparents here. We'll keep striking until the politicians listen."

Many students said they were proud to call the ACT home as the territory closes in on its target of sourcing 100 per cent renewable energy by 2020. Both Victoria and the ACT gave the green light for their public servants to attend but on the eve of the strike Commonwealth staffers were warned by Finance Minister Mathias Cormann to stay in the office.

Some staffers, stowing lanyards hastily in their pockets, confessed they had come down on their lunch hour anyway.

Unions ACT boss Alex White said he was thrilled to support his young daughter in the Canberra leg of the strike as more than 30 unions across the country, including the Australian Education Union, stood with students.

"There are no jobs on a dead planet," Mr White said, stressing the impact of climate change - which falls hardest on the most vulnerable - was already causing a class divide.

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National president of the United Firefighters Union Greg McConville said global warming was also being felt by Australians suffering longer bushfire seasons and extreme weather events like floods and storms.

The Climate Council has warned that firefighting resources will need to double by 2030 but Mr McConville said services were still under-resourced.

"We're hiring more firefighters, and we need to do two intakes a year to catch up," he said. "It's time to take climate change seriously."

The Australian Conservation Foundation, Greenpeace and other charities have praised the student strikes as a "game-changer".