The Paris rally for unity against terrorism is the largest demonstration in France's history.
Calling the rally "unprecedented," the French Interior Ministry says the demonstrators are so numerous they spread beyond the official march route, making them impossible to count.
French media estimate over three million are taking part, more than the numbers who took to Paris streets when the Allies liberated the city from the Nazis in the Second World War.
Ahead of the rally, French President Francois Hollande said: "Today, Paris is the capital of the world.
"The entire country will rise up," he added.
Thousands carrying Tricolore flags and tributes to victims descended on the Place de la Republique for the march.
Dignitaries including Taoiseach Enda Kenny, Foreign Affairs Minister Charlie Flanagan and British Prime Minister David Cameron were among up to 1.5 million people attending the march organised after 17 victims were killed during attacks over three days.
The Taoiseach said "Nous sommes tous Francais aujourd hui" - we are all French today". Mr Kenny is one of 44 world leaders to take part in today's rally in the French capital in the aftermath of the violent attacks by Islamist jihadis this week which left 17 people and the three gunmen dead.
Commenting on reports that Ireland is a hub for jihadist activity, he said, "This matter is being monitored very carefully by the security authorities in Ireland." Mr Kenny was speaking after a meeting with the French President at the Elysee Palace in Paris.
Also taking part in the huge march in solidarity with the shocked citizens are German Chancellor Angela Merkel, Mr Cameron, Israeli prime minister Benjamin Netanyahu and Palestinian Auhtority president, Mahmood Abbas.
Earlier on his arrival in Paris, Mr Kenny reiterated Ireland's support. "Today we march to show that Liberte, Egalite, Fraternity are written, not alone on the history and monuments of the Fifth Republic but in the hearts and minds of the people of France and our European Union," he said.
"Voltaire wrote that ‘tolerance is the consequence of our humanity’. And today we march here in his city to defend that tolerance and humanity against the hatred and extremism that would dismantle and destroy them. In our solidarity we show the agents of such destruction that to us their actions are anathema, their propositions absurd," he added.
All public transport - buses, trains and the Metro was running free of charge in the capital today, to facilitate the enormous crowds expected at the rally. From lunchtime, the Place de la Republique was a sea of French tricolours and placards bearing the now-familiar slogan 'Je Suis Charlie', with the crowd chanting "Charlie, Liberte". Calling on the French people to take part, President Hollande said, "Today, Paris is the centre of the world".
Thousands of police and troops have been deployed to tighten 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 police and soldiers 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.
Meanwhile, a video has emerged showing terrorist Amedy Coulibaly defending the atrocities and swearing allegiance to Islamic State (IS).
The 32-year-old killed a policewoman and four hostages at a Jewish supermarket in separate attacks planned to follow the massacre of 12 people by brothers Said and Cherif Kouachi at the offices of satirical magazine Charlie Hebdo on Wednesday.
In the seven-minute clip, reportedly released on Twitter by IS this morning - two days after Coulibaly's death during a police assault on the kosher store where he had taken several hostages - he describes the strikes as "completely legitimate".
It came as Coulibaly was linked to the shooting of a jogger in a Paris suburb on the same day as the Charlie Hebdo massacres. A prosecutor said ballistics tests on shell cases from the shooting on Wednesday connected them to an automatic weapon used at the Jewish supermarket attacked in the east of the capital two days later.
Coulibaly's girlfriend Hayat Boumeddiene remains on the run, amid reports that she may have fled to Syria before all three killers died in co-ordinated police strikes on Friday.
With France still on high alert, a huge security operation has been deployed to combat the threat of fresh attacks.
More than 5,000 police and soldiers have been mobilised for the rally, with marksmen positioned on surrounding rooftops and security teams scouring drains.
Around 50 heads of state and government are gathering with Mr Hollande at the Elysee Palace before embarking on what is billed an unprecedented show of international solidarity near the head of the march through the capital.
Among those alongside Mr Cameron in a phalanx of senior figures marching behind the families of victims are expected to be Spanish prime minister Mariano Rajoy, German chancellor Angela Merkel, Italian prime minister Matteo Renzi, Ukrainian president Petro Poroshenko - with Russia also represented by foreign minister Sergei Lavrov.
Also attending are Palestinian president Mahmoud Abbas and Israeli premier Benjamin Netanyahu - who has directly encouraged French and other European Jews to flee the threat of terror and move to Israel.
Mr Hollande is due to meet leaders of the Jewish community before the march.
The US will be represented by attorney general Eric Holder, who is in the city along with other security ministers including Mrs May, for talks about the threat posed by Islamist extremism.
Public transport has been made free all day to encourage people to attend without cars.
The now-familiar slogan of solidarity "Je Suis Charlie" - or "I Am Charlie" after the name of the satirical magazine - is to be seen everywhere, from bus shelter posters along the route to the corner of the screens of public television stations.