More than 100,000 people gathered in cities around France as night fell to pay tribute to the 12 people gunned down in an attack against the Charlie Hebdo satirical weekly on Wednesday.

In Paris tens of thousands more gathered at Republique square, not far from where the attack took place, police said. Officials in cities such as Marseille, Toulouse and Lyon also reported thousands gathering in public spaces on the country's darkest day in decades.

Demonstrators wore black stickers marked "Je suis Charlie" (I am Charlie), a slogan aimed at showing solidarity with the victims of the deadliest attack in France in decades.

In a somber address to the nation, Hollande pledged to hunt down the killers and urged the country to come together after the tragedy.

"Let us unite, and we will win," he said. "Vive la France!"

People gather to pay tribute at the Place Royal in Nantes January 7, 2015. REUTERS/Stephane Mahe

Others waved banners with slogans such as "Press freedom has no price" and "Charb mort libre" (Charb died free), a reference to the newspaper's slain editor-in-chief Stephane Charbonnier.

Charbonnier was one of four cartoonists killed in the attack that also left 11 people injured.

A woman raises a pen during a vigil to pay tribute to the victims of a shooting by gunmen at the offices of weekly satirical magazine Charlie Hebdo in Paris, at Trafalgar Square in London January 7, 2015. REUTERS/Stefan Wermuth "It's terrible that these people were murdered. In future, no-one will be able to speak his mind. We have to demonstrate in our thousands," said Beatrice Cano, a demonstrator in her fifties, who was carrying the latest issue of Charlie Hebdo.

Thousands of people gather for a moment of silence to pay their respects to the victims of the deadly attack at the Paris offices of French satirical newspaper Charlie Hebdo, in Lyon, central France, Wednesday, Jan. 7, 2015. AP/Laurent Cipriani People gather in solidarity of the victims of a terror attack against a satirical newspaper, in Paris, Wednesday, Jan. 7, 2015. AP/Thibault Camus A man holds a placard reading "I am Charlie" as he attends a vigil to pay tribute to the victims of a shooting by gunmen at the offices of weekly satirical magazine Charlie Hebdo in Paris at Republique square January 7, 2015. REUTERS/Youssef Boudlal