Children in Australia and the Pacific Islands kicked off a millions-strong global climate strike on Friday – heeding the rallying cry of teen activist Greta Thunberg and demanding adults take action to stave off environmental disaster.

From Sydney to Seoul to Sao Paolo, children will shut their textbooks for what is expected to be one of the largest-ever collective calls to action in a movement fuelled by social media under the hashtag #climatestrike.

More than 5,000 events are planned worldwide, culminating in a massive rally in New York, where 1.1 million students in around 1,800 public schools have been permitted to skip school.

Dubbed the “Fridays for Future” movement, the campaign wants children around the world to take a more active role in persuading adults to treat climate change more seriously.

They are demanding politicians and businesses take the drastic action needed to stop global warming, which scientists warn will lead to environmental catastrophe under current trends.

On the eve of the strikes, Thunberg – a 16-year-old Swede who has become emblematic of generational tensions over management of the planet – insisted solutions were being “ignored” and called on kids to take ownership.

“Everything counts, what you do counts,” she said in a video message to supporters.

As the sun rose above the international dateline, events began in the deluge-threatened Pacific Islands of Vanuatu, the Solomons and Kiribati – where children chanted “we are not sinking, we are fighting”.

They were later joined by tens of thousands of young Australians who made #climatestrike the country’s top trending hashtag as they prepared to rally in Sydney, Melbourne and dozens of other cities and towns.

While some local authorities, schools and Aussie business have encouraged people to take part in the strikes, some have warned that absences must be explained.

Many students remain undeterred.

“We’re here to send a message to people in power – the politicians, showing them that we care and this is really important to us,” 16-year-old Will Connor said in Sydney. “Otherwise we’re not going to have a future.”

Friday’s mass action sets the scene for a range of high-profile climate events in New York. A Youth Climate Summit will take place at the United Nations on Saturday. (AFP)