Protests are being held in cities across Europe after three gunmen shot dead 12 people at the office of the French satirical magazine, Charlie Hebdo.
Eight journalists, including the magazine's editor, and two policemen were among the dead, the BBC reported.
Eleven people have been confirmed as being injured in the attack, four of them seriously.
A manhunt is underway for the three gunmen, who fled by car, and 800 soldiers and an additional 500 police have been deployed on the streets of Paris.
Police have named and published photos of two suspects - Cherif Kouachi, 32, and his 34-year-old brother Said - saying that they were "likely armed and dangerous".
Parts of Paris remain in lockdown and the capital has been put on the highest alert status.
It is believed to be the deadliest attack in France since 1961, when right-wingers who wanted to keep Algeria French bombed a train, killing 28 people.
More than 100,000 people gathered across France to pay tribute to the victims of Wednesday's massacre by Islamist gunmen in Paris, as thousands also rallied in other European cities and the "I Am Charlie" hashtag swept the Internet.
At least 35,000 held vigils in Paris, and some 20,000 people turned out in the French cities of Lyon and Toulouse, police said.
Videographic showing how and where the deadly attack happened
Thousands more took to the streets in cities including Bordeaux and Marseille.
There were also rallies in European cities such as Berlin, London and Lausanne and in Washington and New York in the United States.
Many demonstrators wore black stickers with the words "Je suis Charlie" (I am Charlie), a slogan aimed at showing solidarity with the victims of the deadliest attack in France in decades and in support of the paper's decision to print controversial prophet Mohammed cartoons.
Piles of pens - symbolising freedom of expression - and candles were being laid in memorials across the square at the Place de la Republique in central Paris.
Charlie Hebdo's website, which went offline during the attack, is displaying the single image of "Je suis Charlie" on a black banner.
Other major newspapers are displaying similar banners.
The latest tweet on Charlie Hebdo's account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.
French President Francois Hollande called it a "cowardly murder" and declared a day of national mourning on Thursday (local time).
He said the country's tradition of free speech had been attacked and called on all French people to stand together. "Our best weapon is our unity," Mr Hollande said in a televised address late on Wednesday.
The masked attackers opened fire with assault rifles in the office and exchanged shots with police in the street outside before escaping by car.
They later abandoned the car in Rue de Meaux, northern Paris, where they hijacked a second car.
No group has so far claimed responsibility for the attack but the perpetrators appear to have been highly organised.
Police said the calm, calculated manner of the assault showed they were highly trained, AFP reported.
Witnesses said they heard the gunmen shouting "We have avenged the Prophet Muhammad" and "God is Great" in Arabic ("Allahu Akbar").
The number of attackers was initially reported to be two but French Interior Minister Bernard Cazeneuve later said security services were hunting three "criminals".
He announced that Paris had been placed on the highest alert.
The attack took place during the magazine's daily editorial meeting.
Cartoonist Corinne Rey said the hooded gunmen entered the building after forcing her to enter the code to open the door.
"They said they belonged to al-Qaeda," she said, adding they had spoken in fluent French.
Paris prosecutor Francois Molins told a news conference that once in the building, the gunmen killed a security guard in reception before going upstairs to the newsroom on the second floor.
The first police to arrive at the scene were three officers on bicycles, but they took refuge until more police arrived.
A police car blocked the gunmen's escape route down the narrow street and the gunmen opened fire.
Eyewitnesses described seeing two black-hooded men entering the building carrying Kalashnikovs, with reports of up to 50 shots fired.
They stopped in a residential street nearby and got out of the car to shoot another policeman.
As the officer was lying on the pavement, injured, the gunmen continued shooting.
Footage shot by an eyewitness outside the magazine's office showed two armed men dressed in black approaching a wounded police officer lying on a pavement.
One of the men shoots the officer in the head, before both men are seen running back towards a black vehicle and driving away.
The getaway car was found abandoned - after having crashed into another car a short time later in Rue de Meaux, about three kilometres north of the Charlie Hebdo offices in Rue Nicolas Appert.
The attackers hijacked another car, a grey Renault Clio.
As they drove away, the gunmen are believed to have knocked down several pedestrians.
Pictures of the car showed the windscreen full of bullet holes.
Gilles Boulanger, who works in the same building as the office, told French TV channel Itele: "There were several shots heard in the building from automatic weapons firing in all directions.
"So then we looked out of the window and saw the shooting was on Boulevard Richard-Lenoir, with the police. It was really upsetting. You'd think it was a war zone."
Charlie Hebdo editor Stephane Charbonnier, 47, had received death threats in the past and was living under police protection.
French media named the three other cartoonists killed in the attack as Cabu, Tignous and Wolinski. The attack took place during the magazine's daily editorial meeting.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was firebombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.
US President Barack Obama has condemned the "horrific shooting", offering to provide any assistance needed "to help bring these terrorists to justice".
US Secretary of State John Kerry also pledged support for France and efforts to find the attackers.
All of #France must know that each & every American stands w/you today and always – for freedom/against extremism— John Kerry (@JohnKerry) January 7, 2015
UK Prime Minister David Cameron said in a tweet: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."
He tweeted a picture of himself and German Chancellor Angela Merkel and said that they were united in their condemnation of the attack.
Angela Merkel and I met today. We're united in our condemnation of the horrifying Paris murders. pic.twitter.com/oSM2us6I8I— David Cameron (@David_Cameron) January 7, 2015
The Arab League and the Al-Azhar Mosque, Egypt's top Islamic institution, have also condemned the attack.
A member of the Islamic State insurgent group has praised the attack.
Abu Mussab told the Reuters news agency the raid was revenge for insults against Islam.
"These are our lions," he said. "It's the first drops - more will follow."
After the attack police warned French media outlets to be on alert and pay attention to security.
France was already on the alert for Islamist militant attacks after several incidents just before Christmas.
Cars were driven at shoppers in two cities, Dijon and Nantes, and police were attacked by a man wielding a knife in Tours.
While the French government denied the attacks were linked, it announced plans to further raise security in public spaces, including the deployment of about 300 soldiers.
Danish newspaper Jyllands-Posten, which published cartoons of the Prophet Muhammad in 2005 sparking riots in Muslim countries, said it had stepped up security in the wake of the Charlie Hebdo attack.