Other signs said "We are not afraid" and "I am Charlie" - the message the magazine's survivors placed on its website after the massacre.
To the side of the square, candles were lit and flowers were laid. Shopkeepers put up notices in their windows saying they were "grieving with Charlie Hebdo". Some stores closed early in the evening, as there seemed to be no reason to stay open.
In cities across France crowds rallied - more than 100,000 people nationwide. They gathered in cities as far apart as Rennes in the northwest, Nantes in the west, Lyon in the centre, Bordeaux in the southwest and at the Old Port in the southern city of Marseille. In Paris, staff at news agency AFP held a minute's silence, as did journalists at the daily Le Parisien newspaper. The attack on Charlie Hebdo was an attack on freedom of the press; a cruel warning to cartoonists everywhere.
The worst terrorist attack in modern French history has left a nation united in anger at an assault on press freedom but also fearful of what seems to be a chilling advance in homegrown Islamism.
As a massive manhunt continued across Paris and in Reims, to the city's east, national buildings and media organisations were placed under tight security. The "Vigipirate" anti-terror drill went into higher gear.
Other rallies were staged in London, Berlin, Madrid and Brussels, reflecting a chorus of international outrage voiced by the leaders of allied countries and the UN Security Council.
A vigil for victims of the Paris massacre at Federation Square on January 8, 2015 in Melbourne, Australia. Photo / Getty Images
A small crowd gathers in solidarity during a vigil in Trafalgar Square for victims of the terrorist attack in Paris on January 7, 2015 in London, United Kingdom. Photo / Getty Images
The Catholic diocese of Paris announced that the great bell of Notre Dame cathedral would toll in mourning today, an exceptional tribute. Leftwing parties said they planned a march in Paris this weekend that would be open to all parties espousing the values of the French republic - liberty, equality, fraternity - and to all religions.
In a prime-time address, a sombre President Francois Hollande denounced the bloodbath as "an act of exceptional barbarism" and "definitely a terrorist attack". He declared today to be a day of national mourning, ordered the tricolore to be flown at half-mast for three days and appealed for unity.
"Nothing can divide us. Nothing should divide us," said the President. "Freedom will always be stronger than barbarity."
Mingled with the grief and indignation is brooding concern about the identities of the killers.
According to news reports, the three suspects are French nationals, two of whom have taken part in wars in the Middle East. If confirmed, this would confirm fears, expressed by the French security services, about the danger posed by the return of battle-hardened jihadists. Around 1000 French nationals have gone to fight in Syria and Iraq, according to the security agencies.
So far, homegrown Islamists have been mainly young delinquents of Arab descent who became radicalised in prison and carried out isolated attacks. The most lethal was in 2012, when al-Qaeda-inspired gunman Mohamed Merah killed seven people in Toulouse.
