Thousands of schoolchildren around the world walked out of their classrooms on Friday in what organizers say were the biggest climate change protests to date.

Greta Thunberg, the Swedish schoolgirl who inspired the international movement, joined a crowd of young campaigners as they marched through the streets of the Swedish capital.

The weekly strikes were inspired by a campaign Thunberg launched on social media, Fridays For The Future.

She held her first “School Strike for Climate” in front of the Swedish Parliament building in August.

Tens of thousands of children and teenagers, accompanied by teachers and retirees, have taken part in the protests across the world.

Marches were carried out across Europe on Friday, including in Brussels in Belgium, Zagreb in Croatia and a number of towns and cities across the United Kingdom.

In Brussels one protester was dressed in a polar bear suit while another carried a banner that read: “This is not a test of the emergency climate system this is a real thing. Climate action NOW.”

Organizers of the UK protests YouthStrike4Climate tweeted: “Today's the today. We are set to break records as the biggest youth strike ever seen.

“We are fighting for all life on earth, and we will not stop.”

UK Green Party MP Caroline Lucas wrote in a tweet that the strikes have taken place in 1400 cities, in 100 countries and that around 1.4 million young people have taken part.

“The #fridaysforfuture climate strikes are growing bigger and louder,” she added.

UK Student Climate Network posted on social media: “On May 24th we strike for stronger policy on climate change, better education on climate change and a lower voting age. We strike for our futures.”

There have been weekly protests in hundreds of cities across dozens of countries all around the world.

Thunberg has been nominated as a candidate to receive this year’s Nobel Peace Prize.

If she were to win, she would be the youngest person to receive the prize since Pakistan’s Malala Yousafzai, who was 17 when she won the accolade.

On her Twitter page she describes herself as “a 16-year-old climate activist with Asperger's (syndrome)”.

She also spoke at the UN Climate Talks in Poland in December. EFE-EPA