Around a million people and more than 40 world leaders - including the Taoiseach Enda Kenny - are expected to take part in a rally in Paris, after three days of terror, that left 17 people dead.
It is being organised to remember those killed in the attacks over the past few days.
People are also taking part in marches - in solidarity with the victims of the Paris terror attacks - in Dublin, Cork and Galway today.
The French Ambassador is expected to attend the march through the streets of Dublin, which is getting underway at The Spire at around noon.
There are rolling road closures in city centre, march is taking place for 'Je suis Charlie' now. Just left Spire en route Kildare Street.— An Garda Síochána (@GardaTraffic) January 11, 2015
In Cork, a silent vigil is also getting underway in Daunt Square, with a similar event at the Spanish Arch in Galway.
French interior minister Bernard Cazeneuve said a total of 1,350 military personnel and 2,000 police will be on duty at the Paris rally.
French police have told those planning to attend the march to be "vigilant" and stay tuned for messages from authorities.
Sharpshooters will be positioned on roofs while drains will be scoured in advance of the event, he said.
Mr Cazeneuve said: "There will be a public order plan of exceptional magnitude to make sure that the rally goes well and also to guarantee maximum security.
"French people will express as a mass their attachment to the values of Republican freedoms and freedom of expression after the bloody attacks that the terrorists inflicted."
French media are reporting that the army will protect Jewish schools and synagogues "if necessary".
Mr Kenny will join the French President François Hollande, and other European and world leaders at today's rally.
"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," read a statement from the Taoiseach.
"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.
"The city of Paris has known revolution, occupation terror, old and new.
"Yet today in defiance of history old enemies march here to defend the values of respect, freedom, dignity and tolerance.
"Values, this week so-well recited by the living so-well practised by the dead that even in the depth of darkness they confirm the city of Paris as the City of Light.
"And today as we march through its streets or join in on screens across the world we are, all of us, its citizens we keep and live in its light.
"Je Suis Charlie, nous sommes tous Parisiens.
"Today, I say to President Hollande and his government and to the people of France and of our Union - May our presence and witness here deepen our sense of brotherhood and sisterhood.
"Together, as Europeans, may we nourish our democracy, protect our liberty, cherish our way of life.
"And in the face of terror may our humanity sustain us and renew us.
"May it be as shattering as our sadness and our silence on this January day."