The French Interior Ministry says 3.7 million marched throughout France for an unprecedented rally to show unity in the face of terrorism after attacks that killed 17.

The Interior Ministry said that an estimated 2.5 million marched across France in cities and towns Sunday, and between 1.2 and 1.6 million in Paris. But it said a precise count is impossible given the enormity of the turnout in the capital, which it called unprecendented..

"It's a different world today," said Parisian Michel Thiebault, 70. He was among a crowd wildly cheering police as their vans made their way through the crowd — a sound unheard of at the frequent protests held in France, where police and demonstrators are often at odds.

Their arms linked, more than 40 world leaders headed the sombre procession, setting aside their differences for a manifestation that French President François Hollande said turned the city into "the capital of the world."

Israeli Prime Minister Benjamin Netanyahu stood near Palestinian President Mahmoud Abbas. Ukrainian President Petro Poroshenko and Russian Foreign Minister Sergey Lavrov also marched. Canada's Public Safety Minister Steven Blaney is also in attendance.

In New York, UN Secretary General Ban Ki-moon issued a statement in which he said he stood in solidarity with victims of terrorism across the world, and called on the international community to redouble its efforts to combat terrorism in all its forms.

The deadly attacks on a satirical newspaper, kosher market and police marked a turning point for France that some compared to Sept. 11. In the weeks and months ahead, the cruelty will test how attached the French — an estimated 5 million of whom are Muslims — really are to their liberties and to each other.

"Our entire country will rise up toward something better," Hollande said Sunday.

The aftermath of the attacks remained raw, with video emerging of one of the gunmen killed during police raids pledging allegiance to ISIS militant group and detailing how the attacks were going to unfold. Also, a new shooting was linked to that gunman, Amedy Coulibaly, who was killed Friday along with the brothers behind a massacre at satirical newspaper Charlie Hebdo in nearly simultaneous raids by security forces.

Deafening applause

Children, grandparents, Muslims, Jews, Christians, workers, bosses — all joined together in streets and plazas thronged with crowds throughout eastern Paris.

On Paris's Republic Square, deafening applause rang out as the world leaders walked past, amid tight security and an atmosphere of togetherness amid adversity. Families of the victims, holding each other for support, marched in the front along with the leaders, along with journalists working for Charlie Hebdo, the satirical publication that was the target of the attack that started three days of terror. Several wept openly.

"I Am Charlie," read legions of posters and banners. Many waved editorial cartoons, and the French tricolour and other national flags.

The leaders marched down Voltaire Boulevard — named after the Enlightenment-era figure who symbolizes France's attachment to freedom of expression. One marcher bore a banner with his famed pledge: "I do not agree with what you say, but I will fight to the death to defend your right to say it."

The silent march reflects shock over the worst militant Islamist assault on a European city in nine years. For France, it raised questions of free speech, religion and security, and beyond French frontiers it exposed the vulnerability of states to urban attacks.

At a meeting of international interior ministers in Paris, U.S. Attorney General Eric Holder announced that the White House will host a high-level summit on countering violent extremism on Feb. 18.

Released from custody

The three days of terror began Wednesday when brothers Said and Cherif Kouachi stormed the newsroom of Charlie Hebdo, killing 12 people. Al-Qaeda's branch in Yemen said it directed the attack by the masked gunmen to avenge the honour of the Prophet Muhammad, a frequent target of the weekly's satire. Charlie Hebdo assailed Christianity, Judaism as well as officialdom of all stripes with its brand of sometimes crude satire that sought to put a thumb in the eye of authority and convention.

On Thursday, police said Coulibaly killed a policewoman on the outskirts of Paris and on Friday, the attackers converged. While the Kouachi brothers holed up in a printing plant near Charles de Gaulle airport, Coulibaly seized hostages inside a kosher market. It all ended at dusk Friday with near-simultaneous raids at the printing plant and the market that left all three gunmen dead. Four hostages at the market were also killed.

Five people detained in connection with the three days of bloodshed in France were released from custody, the Paris prosecutor's spokeswoman said earlier Sunday. Family members of the attackers have been given preliminary charges, but prosecutor's spokeswoman Agnes Thibault-Lecuivre said no one remained in detention Sunday over the attacks.

A crowd gathers in Republique square before the demonstration in Paris, France on Sunday. More than 2,200 police are on guard to protect marchers. (Laurent Cipriani/Associated Press)

Early Sunday, police in Germany detained two men suspected of an arson attack against a newspaper that republished the Charlie Hebdo cartoons. No one was injured in that attack.

"The terrorists want two things: they want to scare us and they want to divide us. We must do the opposite. We must stand up and we must stay united," French Foreign Minister Laurent Fabius told French TV channel iTele on Sunday.

France remains on high alert while investigators determine whether the attackers were part of a larger extremist network. More than 5,500 police and soldiers were being deployed on Sunday across France, about half of them to protect the march. The others were guarding synagogues, mosques, schools and other sites around France.

Third shooting linked to kosher market gunman

At an international conference in India, U.S. Secretary of State John Kerry said the world stood with the people of France "not just in anger and in outrage, but in solidarity and commitment to the cause of confronting extremism and in the cause that extremists fear so much and that has always united our countries: freedom."

Posthumous video emerged Sunday of Coulibaly, who prosecutors said was newly linked by ballistics tests to a third shooting — the Wednesday attack on a jogger in a Paris suburb that left the 32-year-old man gravely injured. In the video, Coulibaly speaks fluent French and broken Arabic, pledging allegiance to ISIS and detailing the terror operation he said was about to unfold.

Coulibaly's widow, who has been named as an accomplice, is believed to have travelled to a Turkish city near the Syrian border and then all trace of her was lost, according to a Turkish intelligence official, who was not authorized to speak on the record.