PARIS — More than a million people joined over 40 presidents and prime ministers on the streets of Paris on Sunday in the most striking show of solidarity in the West against the threat of Islamic extremism since the Sept. 11 attacks.

Responding to terrorist strikes that killed 17 people in France and riveted worldwide attention, Jews, Muslims, Christians, atheists and people of all races, ages and political stripes swarmed central Paris beneath a bright blue sky, calling for peace and an end to violent extremism.

The Interior Ministry described the demonstration as the largest in modern French history, with as many as 1.6 million people. Many waved the tricolor French flag and brandished pens in raised fists to commemorate those killed Wednesday in an attack on the satirical newspaper Charlie Hebdo, as well as four others killed at a Jewish supermarket on Friday. Thousands hoisted black and white signs bearing three words that have ricocheted through social media as a slogan of unity and defiance: “Je suis Charlie.”

The crowd included Pascale Trager-Lewis, 45, a lawyer, and her husband, Christian Chevalier, 45, who brought their two daughters because they wanted them to witness a historic event. “We came because my husband is an authentic French person; I am Jewish,” Ms. Trager-Lewis said. “My elder daughter’s godmother is a Muslim, and my closest friend almost became a nun. I came for the Jews who were killed, for the freedom of speech, for religious tolerance.”