French anti-terrorism police converged on an area northeast of Paris on Thursday after two brothers suspected of being behind an attack on a satirical newspaper were spotted at a petrol station in the region.
France's prime minister said on Thursday he feared the Islamist militants who killed 12 people could strike again as a manhunt for two men widened across the country.
Two police sources said that the men were seen armed and wearing cagoules in a Renault Clio car at a petrol station on a secondary road in Villiers-Cotterets some 70 kilometres from the French capital.
Amid French media reports the men had abandoned their car, Bruno Fortier, the mayor of neighbouring Crépy-en-Valois, said helicopters were circling his town and police and anti-terrorism forces were deploying en masse.
"It's an incessant waltz of police cars and trucks," he told Reuters, adding that he could not confirm reports the men were holed up in a house in the area.
A policewoman was killed in a shootout in Paris earlier in the day, but police sources could not immediately confirm a link with Wednesday's killings at the Charlie Hebdo weekly newspaper that marked the worst attack on French soil for decades.
National leaders and allied states described the assault on Charlie Hebdo, known for its lampooning of Islam and other religions as well as politicians, as an assault on democracy. The bells of Notre-Dame cathedral rang out during a minute's silence observed across France and beyond.
Many European newspapers either re-published Charlie Hebdo cartoons or mocked the killers with images of their own.
Montrouge Mayor Jean-Loup Metton said the policewoman and a colleague were attending a reported traffic accident when Thursday's shooting occurred. Witnesses said the assailant fled in a Renault Clio and police sources said he wore a bullet-proof vest and had a handgun and assault rifle.
But one police officer at the scene told Reuters he did not appear to resemble the Charlie Hebdo shooters.
Prime Minister Manuel Valls was asked on RTL radio after an emergency cabinet meeting with President Francois Hollande whether he feared a further attack.
"That question is entirely legitimate, that's obviously our main concern, and that is why thousands of police and investigators have been mobilised to catch these individuals."
'Armed and dangerous'
Police released photographs of the two French nationals still at large, calling them "armed and dangerous": brothers Cherif and Said Kouachi, aged 32 and 34, both of whom were already under watch by security services.
Late Wednesday, an 18-year-old man turned himself into police in Charleville-Mézières near the Belgian border as police carried out searches in Paris and the northeastern cities of Reims and Strasbourg. A legal source said he was the brother-in-law of one of the main suspects and French media quoted friends as saying he was in school at the moment of the attack.
French social media carried numerous reports of police helicopters across northern France. Police tightened security at transport hubs, religious sites, media offices and stores.
There were scattered, unconfirmed reports of sightings of the assailants and police increased their presence at entry points to Paris. One police source talked of a type of "psychosis" setting in with various reports and rumours, but police had to take each of them seriously.
The defence ministry said it had brought in an additional 200 soldiers from parachute regiments across the country to Paris to take the number of military patrolling the capital's streets to 850.
France held a day of mourning for journalists and police officers shot dead by black-hooded gunmen using Kalashnikov assault rifles. French tricolour flags flew at half mast.
Tens of thousands took part in vigils across France on Wednesday to defend freedom of speech, many wearing badges declaring "Je Suis Charlie" (I Am Charlie) in support of the newspaper and the principle of freedom of speech.
Britain's Daily Telegraph depicted two masked gunman outside the doors of Charlie Hebdo saying to each other: "Be careful, they might have pens". Many German newspapers republished Charlie Hebdo cartoons.
The attack raised questions of security in countries across the Western world and beyond. Muslim leaders condemned the shooting but some have expressed fears of a rise in anti-Islamic feeling in a country with a large Muslim population.
France's Muslim Council called on all French Muslims to join the minute of silence and said it was issuing a call for "all Imams in all of France's mosques to condemn violence and terrorism wherever it comes from in the strongest possible way."
Police sources said the window of a kebab shop next to a mosque in the town of Villefrance-sur-Saone was blown out by an overnight explosion. Local media said there were no wounded.
Security services have long feared that nationals drawn into Islamist militant groups fighting in Syria and Iraq could return to their home countries to launch attacks - though there is no suggestion that the two suspects named by police had actually fought in either of these countries.
Britain's Cobra security committee met on Thursday. London's transport network was target of an attack in 2005, four years after 9/11. There have been attacks in countries including Spain, Kenya, Nigeria, India and Pakistan that have raised fears in Europe.
Islamist militants have repeatedly threatened France with attacks over its military strikes on Islamist strongholds in the Middle East and Africa, and the government reinforced its anti-terrorism laws last year.
A total of seven people had been arrested since the attack, he said. Police sources said they were mostly acquaintances of the two main suspects. One source said one of the brothers had been identified by his identity card, left in the getaway car.
Cherif Kouachi served 18 months in prison on a charge of criminal association related to a terrorist enterprise in 2005. He was part of an Islamist cell enlisting French nationals from a mosque in eastern Paris to go to Iraq to fight Americans in Iraq and arrested before leaving for Iraq himself.
The gunmen stormed the journal's offices on Wednesday killing journalists, including its founder and its current editor-in-chief, and shouting "Allahu Akbar!" (God is Greatest). They then escaped in a black car, shouting, according to one witness, that they had "avenged the Prophet".
Charlie Hebdo has published numerous cartoons ridiculing the Prophet Mohammad. Jihadists online repeatedly warned that the magazine would pay for its mockery.
Charlie Hebdo's lawyer Richard Malka said the newspaper would be published next Wednesday with one million copies compared to its usual print run of 60,000.
Satire has deep historical roots in Europe where ridicule and irreverence are seen as a means of chipping away at the authority of sometimes self-aggrandising political and religious leaders and institutions. Governments have frequently jailed satirists and their targets have often sued, but the art is widely seen as one of the mainstays of a liberal democracy.
French writer Voltaire enraged many in 18th century France with caustic depictions of royalty and the Catholic Church. The German magazine Simplicissimus in its 70-year existence saw cartoonists jailed and fined for ridiculing figures from Kaiser Wilhelm to church leaders, Nazi grandees and communists.
"Freedom assassinated" wrote Le Figaro daily on its front page, while Le Parisien said: "They won't kill freedom".
The last major attack in Paris was in the mid-1990s when the Algerian Armed Islamic Group (GIA) carried out a spate of attacks, including the bombing of a commuter train in 1995 which killed eight people and injured 150.