The international group of protesters unfurled banners, spray-painted an anti-nuclear slogan on a wall and waved flags.
Police officials were not available for comment Monday evening, but Pol D'Huyvetter of For Mother Earth said the 21 protesters had not yet been freed.
At 11:30 p.m. GMT Sunday, France detonated the second of a series of explosive devices deep under the Pacific Ocean near its testing site at Mururoa Atoll. The Russian Foreign Ministry on Monday expressed regret at the blast.
"Such actions are counterproductive to the regime of nonproliferation of nuclear weapons and to the drafting of a treaty on the general prohibition of nuclear tests," the Foreign Ministry said, according to Interfax.
France has vowed to sign the Nuclear Test Ban Treaty next year, after the tests are finished.
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