The activists, many dressed in white overalls and carrying sleeping bags, got into the vast Garzweiler lignite mine after a cat-and-mouse game with police, an AFP journalist said.
They ignored earlier police warnings that the site was not safe and a caution from RWE that they would prosecute trespassers.
The activists, using blankets to protect against the sun, were quickly encircled by police and their vehicles to prevent them advancing further. The action was part of a series of protest planned over the weekend.
Garzweiler, which covers 48 square kilometres (18 square mile), supplies lignite, or brown coal, to power stations in the region. The declared aim of the “Ende Gelaende” (EG) protests is to shut down its operations.
On Friday, 500 activists cut off the supply of coal to the Neurath plant, one of Germany's main coal-fired power stations, by sitting down on the rail tracks the supply trains use.
The activists scrawled 'climate justice' on a rock face at the mine. Photo: David Young/DPA
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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.
With this new generation of protesters, the authorities must contend with large numbers of activists who are committed to — and often trained in — non-violent civil disobedience.
A few kilometres from Garzweiler, some 8,000 people protested Saturday in the town of Keyenberg, which is threatened by plans to expand the mine.
“This day is a reason to hope,” said EG spokeswoman Kathrin Henneberger. “Despite the unprecedented failure of the politicians faced with the climate crisis, thousands of people are today sending a clear signal for climate justice.
“Whether it is a demonstration, a school strike or a blockade, this movement is determined to put an end to the era of fossil fuels,” she added.
Police gather near some of the gigantic machinery at the Garzweiler mine. Photo: David Young/dpa
Many of those who took part in the protest were school pupils and students who were part of the “Fridays for Future” demonstrations the day before.
Between 20,000 and 40,000 young activists from 17 countries flocked to Aachen near the Dutch and Belgian borders Friday for a huge show of force of the school-strike movement launched by Swedish teenage activist Greta Thunberg.
This weekend's actions are being closely followed 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.
Many voters agree with the protesters' demand on carbon fuels, to “keep it in the ground”, especially after last year's scorching summer, when drought slashed crop yields, forest fires raged and shipping was halted on dried-out
Germany's decision to abandon nuclear power after the 2011 Fukushima nuclear power station disaster in Japan made it more dependent on coal.
Although the government has committed to abandoning the fossil fuel by 2038, for the climate activists this is judged too little too late.