"The world has become so sick that (being a humorist or cartoonist) has become a dangerous profession!" said a poster carried by one woman in a mass of protesters in Paris' Place de la Republique.
One mourner, Corentin Vacheret, described the slain cartoonists as journalists who "are famous in France, especially among people who value the liberty of expression."
"The fact that they were killed in the most insane circumstances is just sickening," Vacheret said. "I am not a fan of big protests, but I wanted to express that we are not afraid, that this is not going to make us renounce our liberty of expression."
Rallies also unfolded Wednesday night in Tours, Toulouse, Brest, Lyon, Rennes and Poitiers, among other cities in France, long regarded as a cradle of democracy and liberty. They were joined by demonstrations elsewhere in Europe, including London, Barcelona, Berlin and Rome.
The most immediate rallying cry against the attack was "Je Suis Charlie," or "I Am Charlie," in reference to the weekly satirical magazine's title, Charlie Hebdo.
Another sign at the rally in Place de la Republique declared: "I stand up and I express myself with words because they are still the most beautiful weapon!"
According to Le Monde newspaper, French police said as many as 15,000 people gathered in the Place de la Republique, which was closed to traffic.
Demonstrators' candles glowed in the darkness. Marchers were mostly silent for the first hour, Le Monde reporter Maxime Goldbaum tweeted, but then began chanting, "Charlie! Charlie! Charlie!" and "We are Charlie!"
At another point, the chant became a play on words: "Charlie-Charlie-Charliberte!"
One student held a sign at the Paris rally saying, "Long live liberty! Long live France! Long live Charlie!"
Another student, Sacha, told Le Monde newspaper, "We didn't just come here because of the emotion, but because of the principle. Liberty must be defended."
Sruthi Gottipati of Paris posted a photograph on her Twitter page
of someone lighting a flare at the foot of the monument at Place de la Republique, becoming a mirror image of the nearby statues holding torches.
Another photograph showed several people in the crowd holding aloft a series of illuminated letters in the darkness that stated in English: "NOT AFRAID."
Just a few feet away from the main demonstration, the gathering took a more sombre tone. Dozens of people created a shrine of pens and pencils on the paving stones, encircled by tealights and candles.
Many visitors -- some in tears -- stood or sat quietly, with heads bowed, appearing as if they were trying to understand what happened in an office a few streets away.
Faycal Haddad, a Muslim, carried a homemade sign stating "not in my name, not in the name of my religion."
"I don't want anyone to say that my kids, someone from my family, is involved -- this is not Islam," Haddad said. "If I thought that this was what Islam was about, I would not be part of it. No religion in the world can authorize anyone to kill.
"We need to unite against these few people, they are a bacteria, a virus, and we need to find a solution," Haddad added.
He joined the public outpouring on the spur of the moment, straight from work, without telling his wife or kids, he added.
"I felt it was my job to be here," he said. He was planning to bring his family to the protest planned for Saturday in Paris, he said.
Luc Lemoine carried a cartoon of his own making showing a dramatic faceoff between two characters: a masked terrorist with a gun and a balding man with a pencil.
"The drawings never killed anybody," Lemoine said of the Charlie Hebdo cartoonists.
"Like everybody, I was shocked about what happened. Everybody here grew up with Charlie Hebdo. It is part of French culture and has been for many years, so I felt it was important to be here," Lemoine said.
Emmanuel Leger brought his son Binhkim, 15, to a vigil where both held burning candles.
"I think today is the day everything changed," the father said.
"I am an artist, I draw, but just for pleasure, and sometimes my drawings are not politically correct," Leger added. "I wanted my son with me, even if he didn't want to come, I'm sure in several years he will remember this time."
Parisians used social media to urge people to attend the Place de la Republique vigil, which drew a crowd whose numbers reached several thousand. At times, the gathering was silent, but at other moments, many individuals chanted "freedom of expression" loudly.
Participants laid down flowers and lit candles in the cold night. By 10 p.m., the crowds began to thin.
In London, about 700 people gathered at Trafalgar Square in an evening vigil for the French victims.
Many condemned the attackers.
"Shame on you, pens versus gun? There's no competition, is there. Shame on you, shame on you," said one man attending the London event. "It's the most cowardly thing you can do."