People around the world have called on world leaders to take urgent action at the UN Climate Action Summit to avert an environmental catastrophe.
The protests are planned in about 150 countries and were inspired by 16-year-old Swedish activist Greta Thunberg, who will lead the rally in New York.
Organizers said 300,000 people gathered around Australia, the BBC reported.
The start was in Australia, where thousands of students did not enter classrooms today.
The youth gathered in central Sydney and called on large corporations and Australian governments to take urgent action against climate change.
The processions have already started in many European cities, with a total of 150 countries expected.
Greek students holding placards and shouting slogans take part in a "youth strike to act on climate change" demonstration in front of Parliament in Athens, Friday. Thousands of students around the globe, mobilized by word of mouth and social media, skipped class to demonstrate the failure of political elites to take action against global warming.