An extraordinary chain of 1.5 million people, led by a group of world leaders linking arms, marched down the Boulevard Voltaire in a show of force Sunday meant to illustrate the power of unity and freedom of expression over the sting of fanaticism and terror.

After a barrage of violence that traumatized the nation and left 17 victims dead, the boulevards of Paris produced a striking counterimage: French President François Hollande, arm in arm with German Chancellor Angela Merkel and flanked by Israeli Prime Minister Benjamin Netanyahu, Palestinian Authority President Mahmoud Abbas, and a host of European and African leaders.

An estimated 4 million people nationwide — more than a third of them in Paris — mobilized, with sister demonstrations of support erupting from the West Bank to Sydney to Washington. “Paris is the capital of the world today,” Hollande said.

Yet the show of solidarity could not entirely dispel the unease that has followed the attacks.

U.S. Attorney General Eric H. Holder Jr., who was in Paris for a security conference, said on CBS’s “Face the Nation” that the prospect of “lone wolf” terrorist attacks in the United States “frankly, keeps me up at night.”

Israeli leaders urged European Jews to move to Israel to escape anti-Semitism.

But out on the streets of Paris, Christians, atheists, Jews and Muslims stood side by side, sending up shouts of “Charlie, Charlie, freedom of speech!” — a reference to Charlie Hebdo, the satirical newspaper whose offices were attacked by Islamist extremists at the start of three days of terror last week.

Crushing throngs filled the streets with the red, white and blue of the French flag as tearful relatives of the fallen walked in a place of honor on a symbolic 2.1-mile path from the Place de la Republique to the Place de la Nation. A group of Muslims threw white roses from the sidewalks. “Marchers carried a monumental Marianne, the symbol of France and personification of liberty and reason, her robes floating above the crowd.” Authorities called it the largest mass rally in French history.

[Timeline: Three days of terror in France]

Participants purposely marched down the Boulevard Voltaire, the nom de plume of the philosopher of the French Enlightenment who advocated religious tolerance and freedom of expression. It was, many here said, a pushback against religious extremism in a nation that puts secularism first.

“We are here to show that we are not afraid, that we are all French and that we will not be defeated by fear,” said Patrick Bidegaray, a 32-year-old corporate consultant and self-described atheist who attended the march with nine friends, including Christians, Muslims and Jews. “They want to divide us, but we are France. We are the republic, first before everything. We are the republic. Today, we are one.”

Yet even as the marchers spoke of unity, there was also trepidation over a toxic combination of ills laid bare by the attacks, including the increasing specter of homegrown extremism and the growing arm of the militant group known as the Islamic State.

Two hostage situations related to Wednesday's massacre at the satirical newspaper Charlie Hebdo ended Friday evening Paris time when police simultaneously stormed both sites. All three suspects were killed.

The Islamic State championed a video that emerged Sunday in which one of the assailants — Amedy Coulibaly, a 32-year-old of Malian descent — pledges allegiance to the group and claims to have carried out the first attack in the West explicitly in the group’s name. Coulibaly — the only son among 10 children and a petty thief once charged with drug trafficking — shot a policewoman Thursday before taking hostages in a kosher market on Friday and killing four before losing his own life in a police raid. Police killed the two other attackers — brothers Chérif Kouachi and Said Kouachi — on Friday outside a printing plant 26 miles north of Paris, where they had made a last stand.

Yet in the aftermath of the attack, France faces other challenges, too, chiefly growing anti-Semitism and the possibility of a backlash against the Muslim community by the far right. On Sunday, however, the far-right National Front — which won 25 percent of the vote in last year’s local elections and which was purposely snubbed by organizers of the Paris march — failed to draw mass crowds of its own. Roughly 440 miles south of Paris in the city of Beaucaire, Marine Le Pen, the National Front’s leader, held a modest rally of fewer than 1,000 under a banner denouncing “Islamist terrorism.” Some bystanders heckled and booed her.

“Thank you for being here to defend the values of liberty,” Le Pen told the crowd, which had cried out chants of “This is our home!”

There were early signs, however, that elsewhere in Europe — particularly Germany, where an anti-Islamist movement has seen thousands take to the streets of cities nationwide — the Paris attacks were being exploited to swell the ranks of protesters.

Sunday’s Paris march had the feel of a preemptive strike: a bid by France’s mainstream majority not only to stand against Islamist violence, but also to thwart any attempt by the far right to hijack the moment for political gain.

“No Islamic State, No Le Pen,” read one man’s handmade sign.

Still, political opportunism was in the air. Some of the leaders in attendance, such as President Ali Bongo of Gabon, have dubious track records at home on freedom of expression and tolerance.

Netanyahu, meanwhile, appeared to send a message to Jews in France, a country that has seen a surge in anti-Semitic attacks and where a steady stream has begun to immigrate to Israel. He reminded them that the “state of Israel is your home.”

Friday’s attack on the kosher market sent tremors through France’s half-million-strong Jewish community that continue to reverberate. Synagogues have been receiving round-the-clock police protection amid fears of another anti-Semitic strike.

“We’ve been attacked twice — first as Jews, and second as citizens of France,” said Aurelien Kalmucki, a 34-year-old who proudly waved a blue-and-white Israeli flag amid a sea of French tricolors. “Before, we’d tell ourselves that it only happens to others, but now it’s happened to us, and it can happen again.”

At Paris’s Grand Synagogue, 17 candles were lit for the 17 victims. “Why do we always have to be united by tears?” said France’s chief rabbi, Haïm Korsia.

France’s Muslims, too, have been fearful that last week’s violence will only trigger a backlash, with their community ultimately paying the price for the crimes of an extremist few.

Marchers on Sunday said that while all of Paris was represented on the streets, the crowd seemed whiter than the city as a whole, and some wondered why France’s 5 million Muslim citizens might have been underrepresented.

“Maybe they’re scared,” said Sophie Corbeau, 51 and Catholic. “Or maybe they think that somehow, Charlie deserved it.”

Neither applied to her husband. “I’m here because I’m Muslim. And I’m against barbarism,” said Ndiogou Dieng, who immigrated to France from Senegal 35 years ago.

The outpouring found an uncommon collection of world leaders marching arm in arm, making a statement at a time when many nations, particularly in Europe, are grappling with how to handle thousands of their nationals who have gone to fight for the Islamic State. Highlighting that threat, the offices of a German newspaper, the Hamburger Morgenpost, were firebombed early Sunday morning. Following the assault on Charlie Hebdo, it had splashed three of the French publication’s Muhammad cartoons on its front page under the headline “This much freedom must be possible!” There were no reports of injuries.

The United States was represented at the march by Jane Hartley, the U.S. ambassador to France.

Though marchers in Paris mourned all the victims, at the symbolic core of the massive mobilization were the cartoonists of Charlie Hebdo. They pushed the boundaries of political, social and religious satire, wielding their pens in comedic depictions of the prophet Muhammad — a choice that apparently led the Kouachi brothers to storm their offices on Wednesday with assault weapons, killing 12 people. Even the squad cars bearing the French police — themselves a frequent target of Charlie’s sharp wit — displayed signs saying “Je suis Charlie” (“I am Charlie”).

As night fell across Paris, the quiet and reflective mood of the day turned exuberant and joyful. At the Place de la Bastille, a revolutionary patch now marked by a soaring bronze column, a street band belted out a jaunty ragtime tune as a group of young men climbed the monument and unfurled a banner quoting Stéphane Charbonnier, the slain Charlie Hebdo editor: “I’d rather die standing than live on my knees.” The crowd below whistled and cheered.

If the Kouachi brothers’ plan was to silence Charlie Hebdo, they appeared to have failed. Surviving staff members worked feverishly in space lent by the French paper Liberation to put out a special edition Wednesday with some 1 million copies. The ranks of new readers will include Joelle Urvoy, a 64-year-old retired office worker who had never before purchased the irreverent newspaper.

She turned out on Sunday, donning a homemade “Je suis Charlie” pin on her elegant red scarf. “To me, Charlie is now a symbol of freedom of expression, of our right to liberty,” she said. “I have never been a reader, but come Wednesday, I will rise bright and early to be the first person in line when their new edition comes out.”

Virgile Demoustier and Cléophée Demoustier contributed to this report.