People attend a protest as part of the world's largest climate strike in Sydney. (AFP)

Children in Australia and the Pacific Islands kicked off a millions-strong global climate strike 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.

Defiant children

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 told AFP in Sydney.

"Otherwise we're not going to have a future."

This mostly hot and dusty continent finds itself at the sharp end of the climate debate and the changing environment has become a daily fact of life.

People attend a protest as part of the world's largest climate strike in Sydne. (AFP)

Australia is one of the world's largest coal exporters and continues to approve massive new mines that bring jobs and have spurred three-decades of uninterrupted economic growth.

But it has also suffered the compounding effects of climate change. Recent years have brought historic droughts, more intense bushfires, devastating floods and the blanching of the Great Barrier Reef.

Australia's conservative government -- while stopping short of outright climate change denial -- has sought to frame the debate as a choice between jobs or abstract CO2 targets.

Ruling coalition parliamentarian Craig Kelly on Thursday warned children that "everything you're told is a lie".

"The facts are, there is no link between climate change and drought, polar bears are increasing in number," he said.

Businesses taking action

An increasing number of businesses disagree.

"See you on the streets" was the defiant message from Australian pension fund Future Super, which helped muster 2,000 companies behind "not business as usual" pro-strike campaign.

In a similar vein, Amazon chief Jeff Bezos on Thursday pledged to make the US technology and retail giant carbon neutral by 2040 and encourage other firms to do likewise.

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.

UN Secretary-General Antonio Guterres will then host an emergency summit on Monday in which he will urge world leaders to raise their commitments made in the 2015 Paris climate accord.

The agreement saw countries pledge to limit the long-term rise in the average temperature of the Earth to two degrees Celsius over pre-industrial levels, and if possible to 1.5 degrees Celsius.

Thunberg, with her distinctive pigtail, took her message to the US Congress earlier this week to implore the nation's lawmakers to "take real action" to avert environmental disaster.

She also sat down with former US president Barack Obama, and received an Amnesty International award.