At least 3.7 million people including more than 40 world leaders are marching throughout France on Sunday in a rally of national unity to honor the 17 victims of a three-day terror spree that took place around the French capital.

The French Interior Ministry said the rally for unity against terrorism is the largest demonstration in France's history, more than the numbers who took to Paris streets when the Allies liberated the city from the Nazis in World War II.

The ministry said between 1.2 million and 1.6 million marched the Paris streets. But it said a precise account is impossible given the enormity of the turnout in the capital. 

The aftermath of the attacks remained raw, with video emerging of one of the gunmen killed during police raids pledging allegiance to the Islamic State 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.

"Today, Paris is the capital of the world," said French President Francois Hollande. "Our entire country will rise up toward something better."

Israel Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas were among the leaders attending, as were top representatives of Russia and Ukraine. German Chancellor Angela Merkel and British Prime Minister David Cameron were alongside Hollande at the front of the crowd, estimated to be around one million people, Sky News reports.

U.S. Attorney General Eric Holder is in Paris this week to attend a meeting on fighting terrorism, but did not participate in the march.

Before the march began, a moment of silence was observed for the 17 who died in the violence.

As the march progressed through Paris Sunday, small groups sang the French national anthem while giant letters spelling out the word “Why?” were attached to a statue in the Place de la Republique, Sky News reports.

Idriss Nouar, 41, told the Guardian that people took to the streets to seek “closure.”

“For three days we have been glued to our television screens, we cried, we lit candles- we didn’t know what to do or how to explain how we felt,” he said. “Today, here, with the chants, the clapping, the slow walking, it’s a way of expressing our distress and it’s some kind of closure.”

Southeast of Paris, in Saint-Etienne, around 60,000 people joined a march from the city’s rail station to its town hall.

Rallies were also planned in London, Madrid and New York -- all attacked by Al Qaeda-linked extremists -- as well as Cairo, Sydney, Stockholm, Tokyo and elsewhere.

"We are all Charlie, we are all police, we are all Jews of France," Prime Minister Manuel Valls said Saturday, referring to the victims of the attacks that included employees at Charlie Hebdo, shoppers at a kosher grocery and three police officers.

Valls also declared Saturday that France was at war with radical Islam, according to the New York Times.

“It is a war against terrorism, against jihadism, against radical Islam, against everything that is aimed at breaking fraternity, freedom, solidarity,” he said at a speech in Évry, south of Paris.

Leaders from France’s Jewish community said after a meeting with Hollande Sunday that the French leader had told them new security measures would be in place at all Jewish institutions over the next two days.

"We have decided to live our Judaism and we will continue to live normally, as we can't give in to violence", said Roger Cukierman, President of the Representative Council of Jewish Institutions, according to the BBC.

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 honor of the Prophet Muhammad, a frequent target of the weekly's satire. 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.

The bodies of the four French Jews killed at the grocery store will be buried in Israel, Israel's Prime Minister Benjamin Netanyahu said Sunday.

Netanyahu said he had "acceded to the request of the families of the victims of the murderous terror attack" and directed "all the relevant government bodies" to assist in bringing the bodies to Israel. A funeral is tentatively set for Tuesday.

Five people who were held in connection with the attacks were freed late Saturday, leaving no one in custody, according to the Paris prosecutor's office. The widow of the man who attacked the kosher market is still being sought and was last traced near the Turkey-Syrian border.

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.

It was France's deadliest terrorist attack in decades, and the country 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.

"I hope that we will again be able to say we are happy to be Jews in France," said Haim Korsia, the chief rabbi in France, who planned to attend the rally.

"I hope that at the end of the day everyone is united. Everyone, Muslims, Jews, Christians, Buddhists," added Zakaria Moumni, who was at Republique early Sunday. "We are humans first of all. And nobody deserves to be murdered like that. Nobody."

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 the Islamic State group and detailing the terror operation he said was about to unfold.

The Kouachi brothers claimed the attacks were planned and financed by Al Qaeda in Yemen.

The Associated Press contributed to this report.