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 3 days of terror, that left 17 people dead.
It is being organised to remember those killed in the attacks over the past few days.
Several thousand police officers and soldiers are being deployed across the French capital to protect the marchers.
Enda Kenny will join the French President François Hollande, and other European and world leaders, at today's rally.
CBS Paris Correspondent, Elaine Cobbe, is in Paris and she says crowds are already starting to arrive ahead of today's events.
Here at home, people are taking part in marches in Dublin, Cork and Galway.
Eleanor Delaney from France is at the Spire on O'Connell Street in Dublin and she explains why she's come out for today's march
Crowds gather at the Spire on O'Connell St in Dublin. Images courtesy of Sinead Spain.
The Taoiseach Enda Kenny - has released a statement, explaining his reasons for being there:
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.
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.
