NEW YORK — From London to New York City and from Perth to Paris, climate activists are taking part in a global general strike on Friday in what is expected to be the biggest day of climate demonstrations in the planet's history.
Thunberg tweeted: "Incredible pictures as Australia's gathering for the #climatestrike ... Australia is setting the standard!"
According to Swedish schoolgirl Thunberg, who is in New York ahead of the United Nations Climate Action Summit on September 23, around 4,638 events have been organized in 139 countries.
It is not just young people taking part this month, with labor and humanitarian groups, environmental organizations and employees of some of the world's biggest brands also set to participate.
By going on strike on September 20 -- and September 27 in a few countries -- protestors hope to put pressure on politicians and policy makers to act on climate issues.
In New York City 1.1 million pupils will be allowed to skip school on Friday after the city announced it would not penalize public school students joining the strikes, but made it clear that the students did need parental consent.
New York Mayor Bill de Blasio tweeted he supported the move: "New York City stands with our young people. They're our conscience."
Crowds will rally in downtown Manhattan at 12 p.m. ET, where a roster of young climate activists will speak, including Thunberg, who sailed to New York to attend the UN Climate Action Summit.
CNN contributed to this report.