A massive crowd of students has gathered in the Sydney CBD to rally against climate change inaction, with protesters overflowing from Town Hall to the next block.

"We may not have a vote but we have a voice ... and we say climate action now," student speakers told thousands gathered at the Town Hall in the "School Strike 4 Climate" on Friday afternoon.

Thousands of school students attend the global climate strike rally at Town Hall in Sydney. Credit:Janie Barrett

"We're sick of being ignored, we're sick of our futures being turned into political footballs.

"We are striking in every capital city and 40 regional towns and it's not just in Australia, our movement is global, [students in] over 100 countries are striking."