More than a million marchers are expected to take the streets.
Police are seeking accomplices of the gunmen who attacked satirical magazine Charlie Hebdo and a kosher supermarket.
Ahead of the rally a video emerged appearing to show supermarket gunman Amedy Coulibaly pledging allegiance to militant group Islamic State.
In the video he says was working with the Charlie Hebdo attackers, saying: "We have split our team into two... to increase the impact of our actions."
Said and Cherif Kouachi killed 11 people at the magazine offices in the French capital on Wednesday, with a policeman shot dead nearby.
Coulibaly killed four hostages seized at the Hyper Cacher supermarket on Friday. He is also believed to have shot dead a policewoman in Montrouge on Thursday.
Jewish security raised
Sunday's Paris rally is expected to dwarf marches across France on Saturday, when 700,000 people took to the streets.
About 2,000 police officers and 1,350 soldiers have been deployed to protect those taking part.
At the Elysee Palace ahead of Sunday's march, French President Francois Hollande met leaders from the Jewish community.
Jewish leaders said the president had told them new security measures would be put in place at all Jewish institutions on Sunday and Monday.
"We have decided to live our Judaism and we will continue to live normally, as we can't give in to violence", said Roger Cukierman, President of the Representative Council of Jewish Institutions, after the meeting.
Coulibaly is also believed to have shot dead a policewoman on Thursday.
He died when police stormed the supermarket. His partner, Hayat Boumeddiene, is still wanted by police - although she is thought to have fled France last week.
Officials believe she may have entered Turkey en route to Syria.
Interior Minister Bernard Cazeneuve says France will stay on high alert in the coming weeks.
He 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.
Mr Cazeneuve promised "exceptional measures" for the massive unity march in Paris on Sunday, including positioning snipers on roofs.
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).
It will offer France the chance to unite against the violence that began on Wednesday when 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. Eleven people were also injured.
The two were eventually cornered on Friday and shot dead as police moved against them and Coulibaly at the same time.
Stories have been emerging of how ordinary people caught up in the attacks defied the gunmen.
At the supermarket, one Muslim employee hid shoppers in a cold storage room, while the manager of the printworks seized by the Kouachi brothers managed to hide another employee for hours.
Meanwhile, there has been an arson attack at the offices of a German newspaper that reprinted Charlie Hebdo cartoons of the Prophet Muhammad.
No-one was hurt in the assault on the Hamburger Morgenpost in the early hours of Sunday. Two men have been arrested.
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 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.