PARIS—France, joined by world leaders locked arm-in-arm, mounted its largest-ever demonstration Sunday in a defiant, if fragile, display of unity against the terror attacks that tore through its capital last week.

More than three million people, many of different political and religious stripes, marched in rallies across the country. Nearly half of them flooded the streets of Paris, transforming its manicured avenues into rivers of humanity, a stunning turnaround for a city that only days ago was savaged by gunfire and bloodshed.

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