The teenage Swedish activist who decided in August 2018 to leave school on a Friday to strike for climate change said she never imagined the movement would grow so rapidly.
Greta Thunberg, 16, said todays strikes, first in Australia and across the Pacific before spreading across the world, were a victory.
She said the UN, due to meet to discuss the environment crisis should feel ashamed if it failed to take action as hundreds of thousands of people took to the streets as part of the second Global Climate Strike.
Many were children who skipped school to take part following a similar event in March.
Events began in 110 Australia towns and cities, including Sydney and the capital Canberra, where an estimated 300,000 protesters called on the country, as the worlds largest exporter of coal and liquid natural gas, to take drastic action to reduce greenhouse emissions.
It was the biggest protest in the country since the Iraq War in 2003.
Greta has staged weekly demonstrations under the heading Fridays for Future over the past year, calling on world leaders to step up their efforts against climate change.
She said: I would never have predicted or believed that this was going to happen someday and so fast.
She is expected to speak at the UN Climate Action Summit in New York on Monday.
In Berlin, organisers said 80,000 people gathered in front of the Brandenburg Gate near where Chancellor Angela Merkels cabinet was discussing a plan to curb Germanys impact on the environment.
Rallies were also held in the South African cities of Johannesburg and Pretoria, as well as Kenyas capital Nairobi.