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 down the two heavily armed brothers suspected in the massacre to make sure they don’t strike again.

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 the crowd that gathered near Notre Dame cathedral fell silent to honor 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, and police patrolled schools, places of worship and schools. Britain increased its security checks at ports and borders.

Satirical newspaper Charlie Hebdo had caricatured the Prophet Muhammad and witnesses said the attackers claimed allegiance to al-Qaida. One of the suspects on the run had a past conviction for recruiting jihadis to fight in the Mideast.

Two men resembling the suspects robbed a gas station in northeast France on Thursday morning and police swarmed the site while helicopters hovered above. Officials said later the newspaper attackers were not there.

Two explosions hit near mosques early Thursday, raising fears the deadly attack at Charlie Hebdo would ignite 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 near Lyon, southeast of the capital.

But France’s top security official abandoned a top-level meeting after just 10 minutes 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.

French Prime Minister Manuel Valls said the two suspects still at large in the Charlie Hebdo slayings — Cherif Kouachi, 32, and Said Kouachi, 34 — were known to France’s intelligence services. Cherif Kouachi was convicted of terrorism in 2008 for being involved in a network sending radical fighters to Iraq.

The two should be considered “armed and dangerous,” French police said in a bulletin, appealing for witnesses after a fruitless search in the city of Reims, in French Champagne country.

A third suspect, Mourad Hamyd, 18, surrendered at an eastern police station after learning his name was being linked to the attacks in the news, said the Paris prosecutor spokeswoman Agnes Thibault-Lecuivre. She did not specify his relationship to the Kouachi brothers.

A French security official said seven people had been arrested overnight, speaking on condition of anonymity because the investigation was ongoing. He did not elaborate on their possible links to terror.