At the Sydney event, a student organiser told the crowd: "We have three demands for our politicians".
"No new coal, including the Adani mine; 100 per cent renewable energy generation by 2030; and a just transition and job creation for fossil fuel workers and communities," she said.
"Each of these goals are achievable, the only thing that's lacking is political will."
"We are in the grips of a massive global crisis and you are leaving children to clean up your mess ... Where is the justice in that?"
SBS News talked to a group of students who made a sign saying "you'll die of old age, I'll die of climate change".
Sasha Dryza, 12, said, "I don't see any future for us and generations to come because the government doesn't care".
Claudia Cox, 11, also took aim at the Coalition.
"The government is not doing anything about climate change. I'm worried about my future. There might not even be a future."
Fifi Onay, 12, said, "people need to stop talking about climate change and fix it".
Participant Pine Esera of the Pacific Climate Warriors said she was "fighting for the pacific".
"Pacific Islanders are on the forefront of climate change ... We see Australia as a big brother to us and we want that to transform into urgent action."
Former MP Kerryn Phelps was one of many adults who joined the protests with the students.
"I think today will go down in history as a turning point in global action on climate change," she told SBS News.
Chloe Redman attended the Sydney protest with a group of friends.
"We were just discussing that we might not have kids, because of what is happening to the planet," the 26-year-old said.
The Sydney protest was largely peaceful but police arrested one non-participant on the sidelines of the event.
A police spokesperson said the man was "behaving erratically" and is yet to be charged.
At the Canberra event, year six student Eva told SBS News "our planet is important, and if we think our planet is important then we should save it".
"One person can change a lot of things, as we know from Greta Thunberg."
And year eight student Finlay Rochford said: "we're coming together to solve this - and really change the world".
"When we come out here ... We really show the world what's actually happening."
The Australian Council of Trade Unions supported the strike.
"One of the fundamentals of unionism is the power of people joining together to stand up for justice. We have and must take a stand for our future when our government will not," the ACTU said in a statement.
The strike is the latest in a worldwide movement started in August 2018 when 16-year-old Swedish climate activist Greta Thunberg began protesting outside Sweden's parliament on school days.
She'll participate in the UN's youth climate forum on Saturday and address world leaders at the UN secretary general's climate summit on 23 September.
She is currently in the US after taking a yacht across the Atlantic to prevent carbon emissions, and urged US lawmakers to "listen to the scientists".
But some government MPs have voiced disapproval of the strikes.
On Friday, acting Prime Minister Michael McCormack said climate protests should be held on weekends when they would not be disruptive to the community.
"I think these sorts of rallies should be held on a weekend where it doesn't actually disrupt business, it doesn't disrupt schools, it doesn't disrupt universities," Mr McCormack told reporters in Melbourne.
"I think it is just a disruption."
He said students would learn more at school than at a protest rally.
On Thursday, Coalition MP Craig Kelly told the students planning to join the strike that "everything you are told is a lie".
"The facts are, there is no link between climate change and drought, polar bears are increasing in number," Mr Kelly said during Question Time.
"Today's generation is safer from extreme weather than at any time in human history," he said.
The Primary Principal Schools Association's Malcolm Elliott said the protest did raise safety issues.
“There’s not really much need for a strike, although we respect the need for students to express themselves with some sense of freedom and we certainly believe in some democratic processes but what we’re saying if you’re of school age, the place you really should be at school," he said.