Nearly 40,000 Australian students were expected to take the day off school to participate in strikes around the country.
In Sydney, speakers spoke out against rebukes from Prime Minister Scott Morrison and Premier Gladys Berejiklian.
"Our Prime Minister wants us to do more learning and less activism," Danielle Villafana-Pore from Fort Street High School told the crowd.
"If you want less activism, maybe you should remove basic science from the curriculum, or maybe you could do your job."
Lord mayor Clover Moore praised the "fantastic turnout" for the protest on social media, after recording a video message on Thursday encouraging young people to take action.
It is the second school strike in the past few months in Australia. The first, in November last year, was attended by an estimated 15,000 students across the country.
The movement, which began in Sweden in 2018, has gained traction across the world, with students from over 100 countries expected to take part over the next 24 hours.
Parts of the city have been closed off and buses rerouted, with Park Street closed between Elizabeth and George streets. Sections of Elizabeth, Castlereagh, Pitt and Druitt streets are also closed. Roads and bus routes are expected to return to normal at 3pm.