Viersen (Germany) (AFP) - Tens of thousands of climate activists noisily rallied in Germany Friday to demand action against global warming, now one of the hottest issues on the European political agenda.

While anti-coal activists tried to reach and occupy a massive open-cast lignite mine, elsewhere the "Fridays for Future" student movement staged their biggest international rally so far.

At least 20,000 young activists from 17 countries flocked to Aachen near the Dutch and Belgian borders for a huge show of force of the school-strike movement launched by Swedish teenage activist Greta Thunberg.

"Climate justice without borders," read one banner at the head of the colourful and festive rally. Another demanded: "Raise your voice, not the sea level."

Many of the young activists planned at the weekend to join the "Ende Gelaende" (EG) anti-coal protest, whose members were already playing a cat-and-mouse game with police as they tried to reach the Garzweiler mine.

The German phrase "Ende Gelaende" means that something is irrevocably finished -- similar to "end of story" -- which is how the protesters feel about the fossil fuel age.

- 'Government failing' -

"We are unstoppable, another world is possible," the EG protesters chanted as they walked toward the 48 square kilometre (19 square mile) mine where building-sized excavators churn through what resembles a moonscape.

Many of them were temporarily stuck when police shuttered the train station at Viersen near their tent city protest camp to stop them from reaching the mine some 20 kilometres (12 miles) away.

The EG protesters warned that they would try to block coal operations from Friday and through the weekend, when Greenpeace, Campact and other environmental groups were also planning large rallies in the area.

Police warned the protesters that entering the mine area was dangerous, while RWE also cautioned that anyone caught on its property would be prosecuted for trespassing.

The rallies were closely watched in Germany, where surveys suggest global warming is now the public's top concern, and where the Greens party is for the first time polling neck-and-neck with Chancellor Angela Merkel's conservatives.

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Especially since last year's scorching summer -- when drought slashed crop yields, forest fires raged and shipping was halted on dried-out rivers -- many voters agree with the protesters' demand on carbon fuels, to "keep it in the ground".

- 'Going further' -

Germany, Europe's biggest economy, has long promoted clean renewables such as solar and wind while phasing out nuclear power -- but it is still missing its climate goals because of a reliance on cheap coal.

Merkel's government has pledged to phase out coal by 2038 -- a deadline which the protest movement rejects as far too distant given global warming is melting ice caps and glaciers, raising ocean levels and exacerbating extreme weather events.

"Today we are taking the coal phase-out into our own hands because the government is failing to protect the climate," said EG spokeswoman Nike Malhaus.

One of the activists, Doerthe, 19, said she had "always been a good citizen" but was now ready to demonstrate and "go further" in non-violent demonstrations.

"It's exciting and it's great to see that so many people feel the same way. It comforts me because I know I'm not alone."

Another activist, Guenter Wimmer, 76, charged that Merkel "has been patting young protesters on the head, saying 'it's great what you're doing', while continuing with policies that are depriving them of their future".

Wimmer said he was heartened by the passion for change of the youngsters around him.

"This is about our children and our children's children, and it gives me hope."