Millions have marched against terrorism in the largest protests France has ever seen, led by world leaders in Paris uniting in tribute to the victims of Islamist attacks as the crowd cried “Not afraid”.

The French interior ministry says almost four million people took to the streets across the country, with some estimates putting the number in Paris alone at 1.6 million.

At the head of a vast and colourful procession 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 the streets to mourn the victims of three days of terror that began with a slaughter at the offices of satirical magazine Charlie Hebdo on Wednesday 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.

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”.

Emotions ran high in the grieving City of Light, with many people in tears as they came together under the banner of freedom of speech after France’s worst terrorist bloodbath in half a century.

With so many world leaders present, security in the still jittery capital was tight, with police snipers stationed on rooftops and plain-clothes officers among the crowd.

“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 Berlin, Brussels, Istanbul and Madrid and in US, Canadian and Australian cities as well.

The crowd in Paris was also mourning four Jews killed when a gunman stormed a kosher supermarket, after he had earlier gunned down a policewoman.

Israeli Prime Minister Benjamin Netanyahu joined Hollande at Paris’ main synagogue after the march to honour the Jewish victims.

British PM David Cameron, who also marched, predicted Europe would face the threat of extremism “for many years to come”, but his Italian counterpart Matteo Renzi pledged that the continent “will win the challenge against terrorism”.

Before the march, interior and security ministers from the EU and US held emergency talks 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 Paris attacks had a history of extremism and were known to French intelligence services.

The White House, meanwhile, announced it was organising a February 18 summit on how to fight “violent extremism around the world”.