The march, which is to start at 3 p.m. (9.00 a.m. EST) and be made in silence, will be a show of solidarity and also reflect the profound shock felt in France and across the world over the worst Islamist assault on a European city in nine years.
Seventeen people, including journalists and policemen, lost their lives in three days of violence that began with a shooting attack on the satirical newspaper Charlie Hebdo on Wednesday and ended with a hostage-taking at a kosher supermarket on Friday. The three gunmen were also killed.
Security forces will be on the highest alert for the event, which will attended by about 40 heads of state and government.
German Chancellor Angela Merkel, British Prime Minister David Cameron and Italy Prime Minister Matteo Renzi will march with President Francois Hollande. Turkish Prime Minister Ahmet Davutoglu and Israel's Benjamin Netanyahu are also expected to take part.
"It will be an unprecedented manifestation that will be written in the history books," 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.
Meanwhile, Turkish and French sources said that a woman hunted by French police as a suspect in the attacks had left France several days before the killings and is believed to be in Syria.
French police launched in an intensive search for Hayat Boumeddiene, the 26-year-old partner of one of the attackers, describing her as "armed and dangerous".
But a source familiar with the situation said Boumeddiene left France last week and traveled to Syria via Turkey. A senior Turkish official corroborated that account, saying she passed through Istanbul on Jan. 2.
A senior Turkish security official said Paris and Ankara were now cooperating in trying to trace her but that she had arrived in Istanbul without any warning from France.
"We think she is in Syria at the moment but we do not have any evidence about that. She is most probably not in Turkey," the Turkish source said.
Across France on Saturday, hundreds of thousands of people took part in rallies to demonstrate against the attacks. Many people carried signs saying "Je suis Charlie" (I am Charlie) in reference to the magazine where 12 people, including its top cartoonists, were killed on Wednesday.
"I want my child to be born in a better world," Pierine, 29 and heavily pregnant, said at a march in the Mediterranean city of Nice.
On Sunday, public transport in the Paris region was to be free. In a huge security operation, plainclothes policemen were to protect leading personalities with snipers posted on rooftops along the route from Place de la Republique to Place de la Nation.
"Everything will be done to make sure that those who want to come to this meeting can do so safely," Interior Minister Bernard Cazeneuve said.
However, right-wing National Front leader Marine Le Pen, who is expected to receive a boost in the polls due to the attacks,
said her party would shun the Paris demonstration and instead take part in regional marches.
She accused the Socialist government of trying to take advantage of the event to win greater support.
