Some 40 leaders are to go to the rally, expected to dwarf Saturday marches that saw 700,000 take to the streets.
About 2,000 police officers and 1,350 soldiers are being deployed across the French capital to protect marchers.
Police are seeking accomplices of the gunmen who attacked a satirical magazine and a kosher supermarket.
The interior minister says France will stay on high alert in the coming weeks.
French police are still searching for Hayat Boumeddiene, left, gunman Amedy Coulibaly's partner
Bernard Cazeneuve will host a meeting on Sunday morning of fellow interior ministers from across Europe, including the UK's Theresa May, to discuss the threat posed by militants.
Extra security
Mr Cazeneuve promised "exceptional measures" for the massive unity march in Paris on Sunday, including positioning snipers on roofs.
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Damien Grammaticas reports on huge rallies across France on Saturday
The foreign leaders expected to attend the rally include UK Prime Minister David Cameron, German Chancellor Angela Merkel, Palestinian leader Mahmoud Abbas and Israeli Prime Minister Benjamin Netanyahu.
The march, which will be led by relatives of the victims of last week's attacks, will leave Place de la Republique at 15:00 local time (14:00 GMT).
More than a million people are expected to take part, the BBC's Chris Morris in Paris reports.
Thousands walk during a tribute to the victims of the Paris attacks in Toulouse, Southern France, 10 January 2014
Hundreds of thousands took to the streets on Saturday, with placards reading: "I am against racism"
Hayat Boumeddiene, left, and Amedy Coulibaly in images released by police during the kosher supermarket siege in Paris - 9 January 2015
Before the march, President Francois Hollande will meet leaders from the Jewish community, which is still in shock after a gunman killed four people at a kosher supermarket in eastern Paris on Friday.
The gunman, Amedy Coulibaly, is believed to have shot dead a policewoman the day before.
In a separate attack on Wednesday, two brothers raided the offices of the satirical magazine Charlie Hebdo.
Cherif and Said Kouachi killed 12 people - including eight journalists and two police officers - in the attack. Eleven people were also injured.
Coulibaly and the Kouachi brothers were shot dead on Friday after police ended two separate sieges.
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Police are still hunting for accomplices of the three gunmen, including Hayat Boumeddiene, Coulibaly's partner. However, officials in Turkey believe she may have travelled through the country en route to Syria earlier last week.
'Act of barbarity'
Families of the victims spoke out about the attacks on Saturday.
The family of Ahmed Merabet, one of the police officers killed during Wednesday's attack on the Charlie Hebdo magazine, said they were "devastated by this act of barbarity".
Mr Merabet was "Muslim, and very proud of being a police officer and defending the values of the Republic", his brother Malek Merabet said at a news conference.
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Nearly a quarter of a million people held marches in France on Saturday to condemn the attacks, with large crowds gathering in Paris, Orleans, Nice, Pau, Toulouse and Nantes.
During the marches, people held banners that read "I am against racism", "unity", or "I am Charlie" - the latter a reference to the magazine.
Addressing a large gathering outside the kosher supermarket that was targeted, Prime Minister Manuel Valls said: "Today, we are all Charlie, we are all police officers, we are all Jews of France."
He said he had "no doubt that millions of citizens will come to express their love of liberty, their love of fraternity" in Paris on Sunday.
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How the attacks unfolded (all times GMT)
Wednesday 7 January 10:30 - Two masked gunmen enter Charlie Hebdo offices, killing 12 people, including the magazine's editor. Shortly after the attack, the gunmen kill a police officer nearby.
11:00 - Police lose track of the men after they abandon their getaway car and hijack another vehicle. They are later identified as brothers Said and Cherif Kouachi.
Thursday 8 January 08:45 -A lone gunman shoots dead a policewoman and injures a man in the south of Paris. Gunman later identified as Amedy Coulibaly.
10:30 - The Kouachi brothers rob a a service station near Villers-Cotterets, in the Aisne region, but disappear again.
Friday 9 January 08:30 - Police exchange gunfire with the Kouachi brothers during a car chase on the National 2 highway northeast of Paris.
10:00 - Police surround the brothers at an industrial building in at Dammartin-en-Goele, 35km (22 miles) from Paris.
12:15 - Coulibaly reappears and takes several people hostage at a kosher supermarket in eastern Paris. Heavily-armed police arrive and surround the store.
16:00 - Kouachi brothers come out of the warehouse, firing at police. They are both shot dead.
16:15 - Police storm the kosher supermarket in Paris, killing Coulibaly and rescuing 15 hostages. The bodies of four hostages are recovered.
