Hundreds of thousands of people are attending a march through Paris to honour the 17 people killed in attacks which targeted a satirical magazine, a kosher supermarket and police.
Dozens of world leaders are also expected to join Sunday's march in support for France.
Thousands of police and troops have been deployed to beef up security ahead of the march, which comes after rallies across France on Saturday already drew more than 700,000 in support of the victims of the three-day killing spree.
Around 2,200 security personnel will guard the route of the march, which will run three kilometres from the historic Place de la Republique to Place de la Nation in the east of the capital, the interior minister said, with snipers stationed on rooftops.
British Prime Minister David Cameron was the first to arrive and was expected to be joined by German Chancellor Angela Merkel, Palestinian President Mahmoud Abbas, Israeli Prime Minister Benjamin Netanyahu and the king and queen of Jordan. Russia's foreign minister, Sergei Lavrov, was also expected to join the crowds.
Public transport will be free to ease access into and throughout Paris and international train operator Thalys said it was also cutting fares to the French capital on Sunday.
Hours before the march, crowds began to gather in Place de la Republique, a traditional location for demonstrations.
"We're expecting to see a huge public display of support, of people from all sectors of society, including members of the Muslim community," Al Jazeera's Jacky Rowland reported from the square.
"There will be representatives of all political parties, as one of the main messages is that now is time for national unity and that different agendas of political parties should be put aside."
Hunt for accomplice
The three gunmen responsible for the attacks were shot dead by police on Friday, with investigators trying to track down the girlfriend of one of them.
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Despite earlier being described as "armed and dangerous", security sources later revealed that 26-year-old Hayat Boumeddiene was possibly out of France at the time of the killings.
Reports suggest that she left France early January to travel to Syria via Turkey.
Her partner Amedy Coulibaly shot dead a young policewoman on Thursday and then killed four hostages in a siege at a Jewish supermarket in Paris on Friday.
The chain of violence began on Wednesday with an attack on the office of satirical magazine Charlie Hebdo, that has published cartoons of the Prophet Muhammad and other religious figures. In that attack, Cherif and Said Kouachi shot dead 12 people including some of the country's best-known cartoonists.
Cherif told a French media outlet over the phone that he had been sent to carry out the attack by al-Qaeda's Yemeni branch, which has previously launched several attempts to attack western targets.
A senior member of Al-Qaeda in the Arabian Peninsula (AQAP) claimed Wednesday's attack, saying it was an operation to avenge the honour of the prophet.
Security officials have not confirmed the claim but at least one of the Kouachi brothers was known to have been trained by the group in Yemen.
As demonstrators readied to march, European ministers, as well as US Attorney General Eric Holder, met in Paris to discuss security threats.
Sunday's rally "must show the power, the dignity of the French people who will be shouting out of love of freedom and tolerance", Prime Minister Manuel Valls said on Saturday.
"Journalists were killed because they defended freedom. Policemen were killed because they were protecting you. Jews were killed because they were Jewish,'' he said.
"I have no doubt that millions of citizens will come to express their love of liberty, their love of fraternity," he said of the march.
Source: Al Jazeera and agencies