More than a million people flooded Paris on Sunday in an unprecedented rally against terrorism, led by dozens of world leaders walking arm in arm as cries of "Freedom" and "Charlie" rang out.
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 the slaughter of 12 people at the offices of satirical magazine Charlie Hebdo.
"Freedom! Freedom", "Charlie! Charlie!" chanted the vast crowd, in honour of the cartoonists and journalists killed at Charlie Hebdo over its lampooning of the Prophet Mohammed.
The crowd was also marking the death of four Jews killed when an Islamist gunman stormed a kosher supermarket and a policewoman gunned down in cold blood.
Emotions ran high in the grieving City of Light, with many of those marching bursting into tears as they came together under the banner of freedom of speech and liberty after France's worst terrorist bloodbath in more than 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."
The interior ministry said turnout for the Paris rally was "unprecedented" while French television said rallies across the nation were unseen since the 1944 Liberation of Paris from Nazi occupation.
Isabelle Dahmani, a French Christian married to a Muslim, Mohamed, brought their three young children to show them there is nothing to fear.
Their nine-year-old daughter burst into tears watching the news this week, Isabelle said, adding she had asked if "the bad men are coming to our house?"
The grieving 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."
More than a million also rallied in cities outside the capital and marches were held in several cities across Europe, including Berlin, Brussels and Madrid.
Italian Prime Minister Matteo Renzi pledged that Europe "will win the challenge against terrorism". Earlier he had tweeted using the hashtag #jesuischarlie (I am Charlie), which has already been used more than five million times.
US President Barack Obama was represented by Attorney General Eric Holder, who took part in an emergency meeting of interior ministers to discuss the threats from Islamic extremism.
The ministers urged a strengthening of the EU external borders to limit the movement of extremists returning to Europe from the Middle East and said there was an "urgent need" to share air passenger information.
Hollande has warned his grieving country not to drop its guard in the face of possible new attacks.
Ahead of the march, 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 three 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.
German newspaper Bild said the bloodshed in France could signal the start of a wave of attacks in Europe, citing communications by Islamic State leaders intercepted by US intelligence.
France's three days of terror started Wednesday when brothers Said and Cherif Kouachi 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, sparking a manhunt that lasted more than 48 hours.
A day later, a third gunman, Amedy Coulibaly, shot dead a policewoman in a southern Paris suburb.
In a video posted online Sunday, a man who appeared to be Coulibaly said the gunmen coordinated their efforts and claimed he was a member of Islamic State who was avenging attacks by the international community on the extremist group.
The massive hunt for the attackers culminated in twin hostage dramas that gripped the world as 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, the community said.
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 despite being known to authorities for extremism.
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".