Event organisers estimate that about 20,000 people attended the Melbourne protest outside the Old Treasury Building, with the crowd shutting down city streets, blocking traffic and disrupting trams.

Chants of "hey ho, hey ho, fossil fuels they have to go" echoed through CBD streets as thousands of students staged their school strike.

While many of the attendees were too young to vote, their message was political.

They demanded no new coal or gas projects, stopping the Adani coal mine in central Queensland, and moving to 100 per cent renewable by 2030.

Ten-year-old Mimi Vale and her mum travelled for three hours by train to attend the rally.

The Foster Primary School student lives on a drought-ravaged farm in the small town of Toora, Gippsland, and said she's witnessed the impacts of climate change first-hand

There's no grass growing at her family’s beef farm, so Mimi has to help her mother hand feed the cattle.

She was born two weeks before Black Saturday in 2009 and has lived through numerous bushfires.

“If we don’t act fast, the world is going to end,” she said.

The student protests, which drew huge crowds across Australian cities on Friday, have divided politicians.

While Victorian Premier Daniel Andrews said he wouldn’t mind if his three children took part in the rally, federal education minister Dan Tehan said the strikes were orchestrated by professional activists and children were missing valuable class time.

“For action on issues that they think is important, they should do that after school or on weekends," Mr Tehan told reporters ahead of the protests.

One Melbourne private school, Siena College, warned students that if they skipped school to take part in the strike and missed tests, their VCE coursework would be marked as "not assessed".

Some students defied their teachers' orders by attending the rally.

Hazera, a year 9 student from Pascoe Vale Girls College, said one of her teachers told her attending the rally would achieve nothing.

"We came here of our own will," she said.

"We are the future generation who has to live in this world."

Isabella Morgan, a year 9 student from the Academy of Mary Immaculate, said her Catholic school was happy for students to attend the event, as they long as they received signed permission from their parents.

“The earth is melting, it’s heating up, the deserts are becoming hotter,” she said. “So many people are affected.”

She proudly waved a homemade placard that read: ‘will you explain to my children what polar bears were?’

Julian De Marco, a Year 10 student at Balwyn High School, said students had to take a stand against climate change because the government hadn’t.

“Our government is more interested about staying in power than doing something about climate change,” he said.

“We have the solutions, we need to start investing in renewable energy.”

It was the first rally that nine-year-old Charlie Cook had ever attended.

Flanked by his mum Kristen, the Southern Cross Grammar School student said the event was “fun” and had given students a voice.

“I want to stop Adani,” he said.

“We are the children who have to live through this.”

After listening to student speakers and bands on the steps of Old Treasury, the protesters marched along Collins, Russell, Lonsdale and Spring Streets.

It’s the second major strike organised by students – in November thousands of young Australians marched through major cities to call for action for climate change.

The protests were part of the global School Strike 4 Climate campaign which was inspired by 16-year-old Swedish student Greta Thunberg protesting outside the Swedish parliament in 2018.

She was recently nominated for a Nobel Peace Prize for her activism.