PARIS—A week of social unrest at the edges of France’s capital has begun to resonate on the campaign trail as Marine Le Pen of the far-right National Front seized on the violence to rail against the country’s ruling class.

Stone-throwing rioters, smashed storefronts and burning vehicles have become a nightly ritual across Paris’s banlieues, the belt of downtrodden suburbs that surrounds the manicured capital. Police detained 21 people, many of them minors, late Sunday as rioting continued for the seventh straight night.

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