PARIS -- Scattered gunfire and explosions shook France on Thursday as its frightened yet defiant citizens held a day of mourning for 12 people slain at a Paris newspaper. French police hunted two heavily armed brothers suspected in the massacre, fearing they might strike again.
The two suspects -- one a former pizza deliveryman who had a prior terror conviction and a fondness for rap --should be considered "armed and dangerous," French police said in a bulletin.
French President Francois Hollande -- joined by residents, tourists and Muslim leaders -- called for tolerance after the country's worst terrorist attack in decades. At noon, the Paris metro came to a standstill and a crowd fell silent near Notre Dame cathedral to honour Wednesday's victims.
"France has been struck directly in the heart of its capital, in a place where the spirit of liberty -- and thus of resistance -- breathed freely," Hollande said.
France's prime minister said the possibility of a new attack "is our main concern" and announced several overnight arrests. Tensions ran high in Paris, where 800 extra police patrolled schools, places of worship and transit hubs. Britain increased its security checks at ports and borders.
The satirical newspaper Charlie Hebdo had caricatured the Prophet Muhammad and witnesses said the attackers claimed allegiance to al-Qaida in Yemen. Around the world, from Berlin to Bangkok, thousands filled squares and streets for a second day, holding up pens to protect the right to freedom of speech.
"The only thing we can do is to live fearlessly," wrote Kai Diekmann, editor in chief of Bild, Germany's biggest-selling daily. "Our colleagues in Paris have paid the ultimate price for freedom. We bow before them."
Eight journalists, two police officers, a maintenance worker and a visitor were killed in Wednesday's newspaper attack and 11 people were wounded, four of them critically. The publication had long drawn threats for its depictions of Islam, although it also satirized other religions and political figures.
French Prime Minister Manuel Valls said the two suspects still at large in the slayings -- Cherif Kouachi, 32, and Said Kouachi, 34 -- were known to France's intelligence services. Cherif Kouachi was convicted of terrorism in 2008 for ties to network that sends radical fighters to Iraq. His lawyer confirmed Thursday that police tracked down the identities of the brothers because one left his ID behind in a getaway car.
By Thursday afternoon, authorities focused their search around the towns Villers-Cotterets and Crepy-en-Valois northeast of Paris, according to an official with the national gendarme service.
Two men resembling the suspects robbed a gas station in Villers-Cotterets early Thursday, and police swarmed the site while helicopters hovered above. Later large numbers of special police units arrived in Crepy-en-Valois amid reports the suspects had holed up there. However, the gendarme official later said the men had not yet been located.
A third suspect, Mourad Hamyd, 18, surrendered at a police station after hearing his name linked to the attacks, a Paris prosecutor's spokeswoman said. His relationship to the Kouachi brothers was unclear.
One French police official, speaking on condition of anonymity because the investigation was ongoing, said the suspects were linked to a Yemeni terrorist network. And a witness, Cedric Le Bechec, wrote on Facebook that the attackers said as they were fleeing "Tell the media that it's al-Qaida in Yemen."
The governor of a southern province in Yemen told The Associated Press on Thursday that four French citizens had been deported from Yemen in the last four months. Gov. Ahmed Abdullah al-Majidi said he didn't have their names and there was no confirmed link between those deportations and the Charlie Hebdo attack.
Two explosions hit near mosques in France early Thursday, raising fears the deadly attack at Charlie Hebdo was igniting a backlash against France's large and diverse Muslim community. No one was injured in the attacks, one in Le Mans southwest of Paris and another in Villefranche-sur-Saone, near Lyon, southeast of the capital.
France's top security official, meanwhile, abandoned a top-level meeting to rush to a shooting on the city's southern edge that killed a policewoman. The shooter remained at large and it was not immediately clear if her death was linked to Wednesday's deadly attack.
A French security official said seven people had been arrested overnight in the investigation, speaking on condition of anonymity because the investigation was ongoing.
Fears have run high in Europe that jihadis trained in warfare abroad would stage attacks at home. The French suspect in a deadly 2014 attack on a Jewish museum in Belgium had returned from fighting with extremists in Syria; and the man who rampaged in southern France in 2012, killing three soldiers and four people at a Jewish school in Toulouse, received paramilitary training in Pakistan.
Both al-Qaida and the Islamic State group have repeatedly threatened to attack France, which is conducting airstrikes against extremists in Iraq and fighting Islamic militants in Africa.
Charlie Hebdo editor Stephane Charbonnier, one of those slain, was specifically threatened in a 2013 edition of the al-Qaida magazine Inspire. A caricature of Islamic State's leader was the last tweet sent out by the satirical newspaper, minutes before the attack. Its feed has since gone silent.
One witness to Wednesday's attack said the gunmen were so methodical he at first mistook them for an elite anti-terrorism squad. Then they fired on a police officer. Once inside the building, the gunmen headed straight for Charbonnier, killing him and his police bodyguard first. Shouting "Allahu akbar!" as they fired, the killers then called out the names of other employees.
In Tunisia, the birthplace of one of the slain cartoonists, Georges Wolinski, dozens paid homage in a candlelight vigil outside the French ambassador's residence.
"These people were executed at point-blank range just because of drawings -- drawings that didn't please everyone and provoked anger and controversy but still were just drawings," said journalist Marouen Achouri.