Swedish youth activist Greta Thunberg addressed marchers at the "People's Climate March" outside the Danish parliament on Saturday, just one day after young people skipped school in 125 countries to march for the climate.

"It is absolutely crazy that things have gone so far that children feel like they must sacrifice their education to compensate for the inaction of our leaders and most adults," the 16-year-old climate activist said, referring to young people striking on Friday and earlier as part of her Strike for Climate Change movement.

As many as 1.8 million young people skipped school in 2,350 cities to join a global climate strike on May 24, according to a tally by the youth climate movement, Fridays for Future.

Holding signs that said "Don't burn my future," and "12 years to save ourselves", students took to the streets around the world to spread the message, calling on leaders to take action.

Thunberg first started her climate movement in 2018 when she stood on the steps of the Swedish parliament and said she would not go to school because she was on strike for climate.

On Saturday, she addressed marchers in Copenhagen with an equally firm message.

''Why waste precious time arguing who and what should change first?" she asked the crowd. "Everyone and everything needs to change but the bigger your platform the bigger your responsibility, the bigger your carbon footprint, the bigger your moral duty."

The crowd cheered wildly in response.

The strikes coincide with the EU parliament elections and marchers called on EU politicians to focus on climate issues.

Early exit polls show hope for environmental activists.

In Ireland's EU election, the Green Party did far better than expected, becoming the fourth largest in the country. It emerged in exit polls that 90% of Irish voters said the government should prioritise climate change more, according to Irish national broadcaster RTE.

But the climate marches also come amidst a slew of new reports detailing an upcoming onslaught of consequences from climate change.

A new study published Monday said that global sea levels could rise as much as two metres by 2100, a consequence of climate change that would displace over 180 million people.

The UN released a biodiversity report in early May which similarly stated that humans are responsible for nature's decline as one million plant and animal species are threatened with extinction.

Thunberg for her part has been a vocal activist. She was featured by Time Magazine as a "next generation leader" and nominated for the Nobel Peace Prize for her activism on climate.

''Our house is on fire and we should get angry and transform that anger into action, and then act as if your life depended on it. Because it does," Thunberg said Saturday.