Protests have taken place around the world calling for action to avert environmental catastrophe.

From London to Sydney, here's a look at some of the millions of people joining the global youth climate strike.

A 'polar bear' joined children from the Terra Nova secondary school in Cheshire to demand action to stop global warming (Peter Byrne/PA)

A protester at Millbank in London held up a poster of climate activist Greta Thunberg (Gareth Fuller/PA)

Thousands gathered on the unseasonably hot September day in Victoria Park Gardens to warn against global warming (Jonathan Brady/PA)

A younger protester needed a lie-down as handmade posters warned of the need to tackle climate change (Gareth Fuller/PA)

Students at a large protest in Manchester expressed their anger at Donald Trump (Peter Byrne/PA)

There was a big turn-out at the protest in Cambridge (Joe Giddens/PA)

Protesters in the city later held a 'die-in' to warn about the threat to the planet (Joe Giddens/PA)

Young and old also took to the streets of Birmingham (Jacob King/PA)

Protesters in Cullercoats, Tyneside, staged their protest at the seaside (Owen Humphreys/PA)

Pupils from Colaiste Bride School made their point with striking posters at a protest in Dublin (Brian Lawless/PA)

Thousands of miles away in the Philippines, activists displayed their message in front of 'a human globe' formation (Bullit Marquez/AP)

South Korean protesters held portable LED lamps to show the need for renewable energy (Lee Jin-man/AP)

In India, young protesters gathered outside the Ministry of Housing and Urban Affairs in New Delhi (Manish Swarup/AP)

Activists blocked a road in Frankfurt, Germany, during rush hour (Michael Probst/AP)

A large inflatable globe was bounced through the crowd as thousands protested in Sydney (Rick Rycroft/AP)

People shared some of their favorite signs on Twitter as well: