More than a million people have converged on central Paris for a “national unity” rally in solidarity with the victims of the week’s attacks and in support of freedom of the press.
Security forces are on the highest alert for the event, being attended by about 40 heads of state and government.
German chancellor Angela Merkel, British prime minister David Cameron and Italian prime minister Matteo Renzi were marching with French president Francois Hollande.
Taoiseach Enda Kenny is also attending as is Turkish prime minister Ahmet Davutoglu and Israel’s Benjamin Netanyahu.
"It will be an unprecedented demonstration that will be written in the history books," French prime minister Manuel Valls said.
“It must show the power and dignity of the French people, who will cry out their love of liberty and tolerance,” he said.
The march began at 3pm local time (2pm Irish time) and is being made in silence.
Hundreds of thousands turned out for marches and vigils in cities across France on Saturday, including more than 80,000 in Toulouse, 30,000 in Nantes and 22,000 in Nice.
Mr Valls encouraged people to take to the streets of Paris on Sunday, saying the demonstration would show the French people’s “love of freedom and tolerance” and their attachment to republican ideals.
It is the first time since 1990 that a French president has joined a street demonstration. In May of that year, the then head of state, François Mitterrand, took part in a march against racism and anti-Semitism after the desecration of a Jewish cemetery.
In a pattern reminiscent of the fallout from the series of attacks on soldiers and a synagogue in Toulouse in 2012, politicians have largely refrained from seeking to take political advantage of this week’s atrocities. “Our best weapon is unity, the unity of all our citizens in the face of this ordeal,” Hollande said in a televised address to the nation.
Mr Hollande is marching alongside his predecessor Nicolas Sarkozy, whom he defeated in the presidential election in 2012, providing a rare image of unity between the two rivals. All senior figures in Mr Sarkozy's UMP are attending, as are the leaders of smaller parties on the left and right.
Marine Le Pen, leader of the far-right National Front, is not among them. Speaking after a meeting with Mr Hollande at the Élysée Palace this week, Ms Le Pen said the president didn't invite her to the march and that she "regretted" this.
Instead, Ms Le Pen will march in Beaucaire, a town in southern France where her party holds the mayoralty.
Julien Dray, a prominent socialist, had previously said the National Front had "no place" in a republican march. Mr Valls said there could be "no exclusion from national unity" but added that unity must be built around certain values "that are profoundly republican – tolerance, a refusal to associate [Islam with extremism]."
After her meeting at the Élysée, Ms Le Pen said the killings showed Islamic fundamentalism was on the rise in France and that tighter security and border controls were needed. “We have to be able to respond to this declaration of war by Islamic fundamentalism,” she said. Earlier this week, she called for a referendum to reinstate the death penalty.
Ms Le Pen’s father and longtime party leader Jean-Marie intervened in the debate on Saturday by saying “I am not Charlie”, a reference to the mantra - Je Suis Charlie - that has become a national slogan this week.
Mr Le Pen said he was affected by the deaths of "12 French compatriots" in the Charlie Hebdo attack but felt Sunday's protest was being "orchestrated by the media".
He said the satirical magazine was “an enemy” of the National Front and had only recently called for the party’s dissolution.