In addition to all of its 11 channels, TV5Monde also lost control of its social media outlets and its websites, director Yves Bigot said in a video message posted later on Facebook
On a mobile site that was still active, the network said it was "hacked by an Islamist group." ISIS
logos and markings appeared on TV5Monde social media accounts. But there was no immediate claim of responsibility by ISIS or any other group.
Later Thursday, when it finally regained control of its website, the network posted that it had been the "victim of piracy unprecedented in the world history of broadcasting."
The attack began around 10 p.m. (4 p.m. ET). The screens of the 11 networks went dark.
The network said a message written in three languages -- French, Arabic and English -- was then posted on one of its social media sites. The message, the network said, included threats to the network and "a glorification of Sharia."
The company email went down, as well.
"Everything was done in a very synchronized manner," said Helene Zemmour, the network's digital director. "Shortly after the beginning of the attack, our internal computer system fell, and other programs followed."
French officials promise 500 new jobs to fight cybercrime
Appalled French government officials condemned the attack as an unprecedented ratcheting up of cybercrime and pledged swift and dramatic action.
Foreign Minister Laurent Fabius said the attack was the work of terrorists and called it "an attack on a new scale." He urged all TV networks to be on guard.
"All measures are being implemented, both at the judicial and technical level, and an investigation is underway," he said.
Interior Minister Bernard Cazeneuve said his department would create 500 new jobs, including some for technicians specializing in fighting cybercrime.
"So there is a complete mobilization on the technological, legal and human levels," he said.
And officials pledged to pursue the perpetrators and bring them to justice.
Late Thursday afternoon, it appeared that the network was still grappling with the problem. It had regained control of its Facebook page earlier in the day, and its website -- many pages of which bore an "under maintenance" inscription for much of the day -- somewhat later on.
But it was unclear how many of the broadcast channels the network had recovered. BFMTV, a CNN affiliate in France, was still reporting late in the day that only one of the network's 11 channels was functioning. The network's website did not address the issue.
TV5Monde offers round-the-clock entertainment and news programming that reaches 260 million homes worldwide, according to the Ministry of Culture and Communications.
It functions under a partnership among the governments of France, Canada and Switzerland, as well as the Wallonia-Brussels Federation. Other networks that provide content to TV5Monde include CNN affiliates France 2 and France 3, France 24 and Radio France International.
Yves Bigot, TV5Monde's director, said the network was up to date and secure, which only heightened the sophistication needed to breach its defenses.
"It's been a very powerful attack, because we have very strong firewalls which had been checked -- and that had been checked very recently -- and were said to be very safe," Bigot said. "So obviously it's a very knowledgeable and powerful cyberattack."