French President Francois Hollande is surrounded by head of states. World leaders including Muslim and Jewish statesmen linked arms to lead more than a million French citizens through Paris in an unprecedented march to pay tribute to victims of Islamist militant attacks.
Commentators said the last time crowds of this size filled the streets of the capital was at the Liberation of Paris from Nazi Germany in 1944, reuters reports.
President Francois Hollande and leaders from Germany, Italy, Turkey, Britain as well as Israel and the Palestinian territories moved off from the central Place de la Republique ahead of a sea of French and other flags.
Seventeen people, including journalists and police, were killed in three days of violence that began on Wednesday with a shooting attack on the political weekly Charlie Hebdo, known for its satirical attacks on Islam and other religions.
Giant letters attached to a statue in the square spelt out the word Pourquoi?" (Why?) and small groups sang the "La Marseillaise" national anthem.
"Paris is today the capital of the world. Our entire country will rise up and show its best side," Hollande said.
At least 3.7 million people took part in silent marches throughout the country, the biggest public demonstration ever registered in France. A total of 1.2 million to 1.6 million marched in Paris and a further 2.5 million in other cities, the Interior Ministry said.
The marches mostly proceeded in a respectful silence, reflecting shock over the worst militant Islamist assault on a European city since 57 people were killed in an attack on London's transport system in 2005.
The attackers, two French-born brothers of Algerian origin, singled out the weekly for its publication of cartoons depicting and ridiculing the Prophet Mohammad. The bloodshed ended on Friday with a hostage-taking at a Jewish deli in which four hostages and the gunman were killed.
Some 2,200 police and soldiers patrolled Paris streets to protect marchers from would-be attackers, with police snipers on rooftops and plain-clothes detectives mingling with the crowd.
German Chancellor Angela Merkel, British Prime Minister David Cameron and Italian Prime Minister Matteo Renzi were among 44 foreign leaders marching with Hollande.
Merkel walked to Hollande's left and at his right was President Ibrahim Boubacar Keita of Mali, a country where France intervened to fight Islamist rebels two years ago to the day.
In a rare public display of emotion by the leaders of two powers, Hollande embraced Merkel, her eyes shut and forehead resting on his cheek, on the steps of the Elysee before they headed off to march.
Renzi said the fight against terrorism will be won by a Europe that is political, not just economic.
"The most important is the Europe of values, of culture, of ideals and that is the reason we are here," Renzi said.
Israel's Benjamin Netanyahu - who earlier in the day encouraged French Jews to emigrate to Israel - and Palestinian President Mahmoud Abbas were also present and walked just a few steps from one another.
"In the same way that the civilized world stood today with France against terror, so it must stand with Israel against terror," Netanyahu said at a ceremony in a Paris synagogue.
After world leaders left the march, Hollande stayed to greet survivors of the Charlie Hebdo attack and their families, while hundreds of thousands of people marched slowly and in near-total silence through Paris streets.
"We're not going to let a little gang of hoodlums run our lives," said Fanny Appelbaum, 75, who said she lost two sisters and a brother in the Nazi concentration camp at Auschwitz. "Today, we are all one."
Zakaria Moumni, a 34-year-old Franco-Moroccan draped in the French flag, agreed: "I am here to show the terrorists they have not won - it is bringing people together of all religions."
The attacks have raised difficult questions of free speech, religion and security, and exposed the vulnerability of states to urban attacks.
The head of France's 550,000-strong Jewish community, Roger Cukierman, said Hollande had promised that Jewish schools and synagogues would have extra protection, by the army if necessary, after the killings. He also called for limits on hate speech and more control on suspected jihadists.
Hours before the march, a video emerged featuring a man resembling the gunman killed in the kosher deli. He pledged allegiance to the Islamic State insurgent group and urged French Muslims to follow his example.
Two of the gunmen had declared allegiance to al Qaeda in Yemen and a third to the militant Islamic State. All three were killed during the police operations in what local commentators have called "France's 9/11", a reference to the September 2001 attacks on U.S. targets by al Qaeda.
Interior Minister Bernard Cazeneuve said that at a meeting in Paris on Sunday European interior ministers had agreed to boost cooperation to thwart further militant attacks.
He called for the creation of a European database of airplane passenger names and said Europe should fight against abusive use of the Internet to spread hate speech.
While there has been widespread solidarity with the victims, there have been dissenting voices.
French social media have carried comments from those uneasy with the "Je suis Charlie" slogan interpreted as freedom of expression at all cost. Others suggest there was hypocrisy in world leaders whose countries have repressive media laws attending the march.
Far-right National Front leader Marine Le Pen, whom analysts see receiving a boost in the polls due to the attacks, said her anti-immigrant party had been excluded from the Paris demonstration and would instead take part in regional marches.
Less than 1,000 people gathered in the National Front-ruled southern town of Beaucaire.