The break-in happened at the Tricastin nuclear plant early Monday, Euronews.com reported.
The activists, who were French, Italian, Spanish and Romanian nationals, projected images and slogans onto the plant's chimneys, such as "Tricastin: a nuclear accident" and "[President] Francois Hollande: president of a catastrophe?"
France's interior ministry confirmed that 21 people were arrested after the incident on trespassing charges.
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