Paris (AFP) - More than 700,000 people on Saturday took to streets across France in tribute to the 17 people killed in three days of violence by Islamist extremists, the interior minister said.

From Nice and Marseilles in the south to Besancon in the east and Lille in the north, people poured onto the streets to express their solidarity following Wednesday's attack on the satirical weekly Charlie Hebdo that left 12 dead. The massacre was followed by the fatal shooting of a police officer on Thursday and the murder of four hostages during a siege at a Jewish supermarket on Friday.

"700,000 people have marched" in cities around France, Bernard Cazeneuve told reporters on the eve of a huge Paris rally planned due to be attended by a string of world leaders.

Cazeneuve did not give a detailed breakdown of the figure, but according to an AFP tally of demonstrations many thousands turned out across the country.

In the southern city of Toulouse, police said around 80,000 people took part in a march, with the "enormous" procession stretching up to two kilometres (1.2 miles), according to an AFP journalist.

"Live together, free, equal and in solidarity," read the banner behind which at least 30,000 people also marched in the western city of Nantes.

In Marseille, 45,000 people expressed similar sentiments with a rally banner that said "For democracy, equality, freedom, let's fight fascism".

Individual marchers held up placards with the words "Not Afraid!"

In Pau, a further 30,000 to 40,000 people staged a silent march with school pupils leading the way holding a banner emblazoned with the words: "We are all Charlie".

"It's a great popular movement... it's beautiful and significant, infinitely precious," the southwestern city's mayor Francois Bayrou told AFP.

In Besancon, another 20,000 took to the streets, an AFP correspondent said, while in northern Orleans around 22,000 rallied, according to a police source.

In Nice, at least 23,000 demonstrators were counted, police sources said, in a demonstration which snaked for around a kilometre along the famous seafront Promenade des Anglais, ending at the war memorial where a wreath was laid in the presence of representatives of different faiths.

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A further 22,000 people turned out in northern Lille and thousands more in several other towns and cities across France.

Outside France, several thousand people, including many Muslims, rallied in Milan with many carrying signs that said "I am Charlie" or "Not In My Name".

The rallies come ahead of a march expected to draw up to a million people on Sunday in which French President Francois Hollande will be joined by a host of world leaders including German Chancellor Angela Merkel and British Prime Minister David Cameron.

Others due to participate include Russian Foreign Minister Sergei Lavrov, Turkish Prime Minister Ahmet Davutoglu and US Attorney General Eric Holder.

The prime ministers of Belgium, Denmark, Italy, Spain and The Netherlands will also attend along with the presidents of Mali, Niger and Ukraine.

Hollande will also be joined by NATO Secretary General Jens Stoltenberg, president of the European Union Donald Tusk and European Commission president Jean-Claude Juncker.

"This will be an extraordinary demonstration... which must show the power and the dignity of the French people who are going to proclaim their love of freedom and tolerance," French Prime Minister Manuel Valls said.

Ahead of that mass rally in Paris, hundreds of troops were on Saturday deployed around the city, which is already on the highest possible alert.

Following a crisis meeting, the French government announced that all necessary measures would be taken to ensure the safety of those attending.

The three-day killing spree by three Islamic militants ended on Friday following a massive police operation.

All three gunmen died in final confrontations with police.