More than a million people thronged the streets of Paris Sunday in the biggest rally in French history, led by dozens of world leaders walking arm-in-arm as cries of "Freedom" and "Charlie" rang out across the country.
The interior ministry said 3.7 million people took to the streets nationwide, with Paris alone seeing an "unprecedented" 1.5 million demonstrators.
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 Paris' iconic streets to mourn the victims of the three days of terror that began with a slaughter at the offices of satirical magazine Charlie Hebdo 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.
Emotions ran high in the grieving City of Light, with many of those marching in tears as they came together under the banner of freedom of speech after France's worst terrorist bloodbath in half a century.
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."
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 watching TV pictures of heavily armed 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 mourning families of those who died in the shootings led the march, alongside the representatives of around 50 countries.
Patrick Pelloux, a Charlie Hebdo columnist, fell sobbing into the arms of Hollande in an emotional embrace.
With dozens of world leaders present, security in the jittery French capital was beefed up, with police snipers stationed on rooftops and plain-clothes officers among the crowd in a city still reeling from the Islamist attacks.
"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 and marches were held in several European capitals , including Berlin, Brussels and Madrid.
The crowd in Paris was also mourning four Jews killed when an Islamist gunman stormed a kosher supermarket, after earlier gunning down a policewoman.
Israeli Prime Minister Benjamin Netanyahu joined Hollande at Paris' main synagogue after the march to honour the victims.
'We will win'
British Prime Minister David Cameron predicted Europe would face the threat of extremism "for many years to come", but his Italian counterpart Matteo Renzi pledged that Europe "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 met 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 attacks had a history of extremism and were known previously to French intelligence.
Hollande has warned his traumatised country not to drop its guard in the face of possible new assaults.
He met representatives from the Jewish community who said authorities had agreed to deploy soldiers to protect Jewish schools and synagogues in France "if necessary."
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.
Burials in Israel
France's three days of terror started Wednesday when the Kouachi brothers burst into Charlie Hebdo's offices in central Paris and sprayed bullets into the 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.
In a video posted online Sunday, a man who appeared to be Coulibaly said the gunmen had coordinated their efforts.
The massive hunt for the attackers culminated in twin hostage dramas that gripped the world.
Coulibaly stormed into a Jewish supermarket in eastern Paris and seized terrified shoppers.
The two brothers took one person 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.
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.
The attacks have raised mounting questions about how the gunmen could have slipped through the net of the intelligence services.
Coulibaly's mother and sisters condemned his actions.
"We absolutely do not share these extreme ideas. We hope there will not be any confusion between these odious acts and the Muslim religion," they said.
French Prime Minister Manuel Valls admitted there had been "clear failings" in intelligence after it emerged that the brothers had been on a US terror watch list "for years".