Millions marched against terrorism Sunday in the largest protests France has ever seen, led by world leaders in Paris uniting in tribute to the victims of Islamist attacks as the crowd cried "Not Afraid".

The interior ministry said nearly four million people took to the streets across the country, with some estimates putting the number in Paris alone at 1.6 million.

At the head of a vast and colourful procession in the capital, President Francois Hollande linked arms with world leaders, including the Israeli prime minister and the Palestinian president, in an historic display of unity.

A sea of humanity flowed through the streets to mourn the victims of three days of terror that began with a slaughter at the offices of satirical magazine Charlie Hebdo on Wednesday and ended with 17 dead.

The vast crowd chanted "Charlie, Charlie", in honour of the cartoonists and journalists killed at Charlie Hebdo over its lampooning of the Prophet Mohammed.

The crowd brandished banners saying "I'm French and I'm not scared" and, in tribute to the murdered cartoonists, "Make fun, not war" and "Ink should flow, not blood."

Emotions ran high in the grieving City of Light, with many people in tears as they came together under the banner of freedom of speech after France's worst terrorist bloodbath in half a century.

Isabelle Dahmani, a French Christian married to a Muslim, Mohamed, brought the couple's three young children to show them there is nothing to fear.

Their nine-year-old daughter had burst into tears as she watched TV pictures of heavily armed Islamist brothers Said and Cherif Kouachi attacking the magazine's offices, Isabelle said, recalling she had asked if "the bad men are coming to our house?"

The victims' mourning families played a prominent role in the march, alongside representatives from around 50 countries.

In an emotional scene, Charlie Hebdo columnist Patrick Pelloux fell sobbing into the arms of Hollande.

With so many world leaders present, security in the still jittery capital was tight, with police snipers stationed on rooftops and plain-clothes officers among the crowd.

"Today, Paris is the capital of the world," Hollande said. "The entire country will rise up."

Hundreds of thousands of people turned out in other French cities including Bordeaux and Lyon and marches were held in Berlin, Brussels, Istanbul and Madrid and in US and Canadian cities as well.

The crowd in Paris was also mourning four Jews killed when a gunman stormed a kosher supermarket, after he had earlier gunned down a policewoman.

Israeli Prime Minister Benjamin Netanyahu joined Hollande at Paris' main synagogue after the march to honour the Jewish victims and praised the "very firm position" taken by French leaders against what he called "the new anti-Semitism and terrorism" in France.

'We will win'

British Prime Minister David Cameron, who also marched, predicted Europe would face the threat of extremism "for many years to come", but his Italian counterpart Matteo Renzi pledged that the continent "will win the challenge against terrorism".

Earlier Renzi had tweeted using the hashtag #jesuischarlie (I am Charlie), which has been used more than five million times.

Before the march, interior and security ministers from the European Union and the United States held emergency talks to discuss Islamic extremism.

They urged a strengthening of the EU external borders to limit the movement of extremists between Europe and the Middle East and said there was an "urgent need" to share air passenger information.

All three of the gunmen in the Paris attacks had a history of extremism and were known to French intelligence services.

Hollande has warned his traumatised nation to keep up its guard in the face of possible new assaults.

The rampage by the gunmen who claimed to be members of the Al-Qaeda and Islamic State extremist groups was followed by a chilling new threat from the Yemen-based Al-Qaeda in the Arabian Peninsula that France faced more attacks.

Hollande met representatives from the Jewish community who said authorities had agreed to deploy soldiers to protect Jewish schools and synagogues in France "if necessary."

US Attorney General Eric Holder, also in Paris, said there was no "credible information" as yet that Al-Qaeda was behind the attacks in France.

The White House, meanwhile, announced it was organising a February 18 summit on how to fight "violent extremism around the world."

The summit would highlight US and international efforts to prevent the radicalisation and recruitment of followers in the United States and other countries by extremist groups, it said.

Burials in Israel

France's three days of terror started when the Kouachi brothers burst into Charlie Hebdo's offices in central Paris and sprayed bullets into an editorial meeting, killing some of France's best-known cartoonists.

They then slaughtered a Muslim policeman as he lay helpless on the ground before fleeing in a car, sparking a manhunt that lasted more than 48 hours.

A day later, a third gunman, Amedy Coulibaly, shot dead a policewoman in a Paris suburb.

The massive hunt for the attackers culminated Friday in twin hostage dramas that gripped the world.

Coulibaly stormed into a Jewish supermarket in eastern Paris and seized terrified shoppers.

After exchanging fire with police, the two brothers took an employee hostage in a printing firm northeast of Paris. After a tense stand-off police shot them dead as they charged out of the building all guns blazing.

Moments later, security forces stormed the kosher supermarket in eastern Paris, killing Coulibaly but making the grisly discovery that four innocent Jews had died during the hostage-taking.

All four will be buried in Israel on Tuesday.

In a video posted online Sunday, a man who appeared to be Coulibaly said the three gunmen had coordinated their attacks.

Investigators have been trying to hunt down Coulibaly's partner, 26-year-old Hayat Boumeddiene, but a security source in Turkey told AFP she arrived there on January 2, before the attacks, and has probably travelled on to Syria.

[INFOGRAPHIC]: WHAT HAPPENED IN PARIS

'Clear failings'

The shootings have raised questions about how the gunmen slipped through the net of the intelligence services.

French Prime Minister Manuel Valls admitted there had been "clear failings" in intelligence after it emerged that the Kouachi brothers had been on a US terror watch list "for years".

Coulibaly's mother and sisters condemned his actions, saying "we hope there will not be any confusion between these odious acts and the Muslim religion."