French President Francois Hollande and leaders including Israeli Prime Minister Benjamin Netanyahu and Palestinian president Mahmud Abbas marched at the front of the mammoth procession, which began near where gunmen killed 12 people at satirical magazine Charlie Hebdo. (See Pics: World Leaders Take Part in March for Paris Attack Victims)
A sea of humanity flowed through Paris' iconic streets, breaking into applause and spontaneous renditions of the national anthem, as a shell-shocked France mourned the victims of three days of bloody violence.
"Charlie! Charlie!" chanted the vast crowd, 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 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. (Bodies of French Jews killed in Paris Grocery Store to Be Buried in Israel: Benjamin Netanyahu)
Lassina Traore, a 34-year-old French-born Muslim from the Ivory Coast, said the march is "a real sign of how strong France is. It shows that France is strong when she is united against these people."
Netanyahu and Abbas' presence was a stark demonstration of such unity, as they honoured victims that included four Jews at a kosher supermarket and a Muslim police officer.
The leaders observed a minute's silence as the march got underway.
On the streets, many came with their families. Jean-Alain said he brought his seven-year-old son Alessandro with him "so it's more concrete for him, so that he can see that we all think the same thing."
"The people who pick up a gun and kill people are cowards," the 39-year-old gently explained to his boy.
"I want to show that we're not scared of the extremists. I want to defend freedom of expression," said 70-year-old Jacqueline Saad-Rouana.
The families of those who died in the shootings led the march, alongside heads of state and royalty.
Security in the 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," he told ministers before the march.
The king and queen of Jordan were present alongside a host of top European leaders, including German Chancellor Angela Merkel and British Prime Minister David Cameron.
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.
Europe will "win the challenge against terrorism," pledged the Italian leader.
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 European air passenger information. ('Urgent Need' to Share European Air Passenger Information, Say Ministers)
Speaking on a visit to India, US Secretary of State John Kerry said: "We stand together this morning with the people of France. We stand together not just in anger and outrage but in solidarity and commitment in confronting extremists."
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 even deploy soldiers to protect Jewish schools and synagogues "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.
AQAP top sharia official Harith al-Nadhari warned France to "stop your aggression against the Muslims" or face further attacks, in comments released by the SITE monitoring group.
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. (Spain Wants to Change Schengen Rules to Thwart Islamist Returnees)
'Armed and dangerous'
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, 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.
France's bloodiest attacks for more than half a century have raised mounting questions about how the gunmen could have slipped through the net of the intelligence services.
Coulibaly's mother and sisters on Saturday 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.
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".