French police were grilling a suspected Islamist Saturday about an attack in which the man displayed his boss's severed head, as it emerged he may have sent a "selfie" of the decapitation.

Sources close to the investigation said the suspect, Yassin Salhi, a 35-year-old married father-of-three sent a picture of him with the severed head via the WhatsApp messaging service.

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.

With the gunmen still on the loose in Paris after killing 12 people, defiant crowds swarmed into the Place de la Republique, barely a kilometer (half a mile) from the scene of the bloodbath at the Charlie Hebdo satirical weekly.

French police have arrested a young soldier on suspicion of planning to fire shots at a mosque near the eastern city of Lyon, the interior ministry said Sunday.

The 23-year-old man, who investigators say is "close to the radical far-right", was detained at his airbase near Lyon.