Just when France was still scrambling to pick up the pieces following the Islamic State (ISIS) terrorist attacks in its capital, Britain found itself under attack from ISIS hackers in response to a declaration of war made by Anonymous, the shadowy group that earlier vowed to silence extremist propaganda and erase ISIS from the net.

The "sustained" ISIS terrorist cyberattacks were launched late Wednesday night and continued throughout the day, according to Mirror Online, citing the hacker group.

The report said the ISIS supporters targeted Britain in revenge for an Anonymous campaign called OpParis, which has leaked details of a least five men it claims are recruiters for the terror group, as well as taking down 5,500 Twitter propaganda accounts used by the terrorist group.

"Britain had been lit up by the digital blitzkrieg,'' Anonymous posted on its Twitter account set up to publicise anti-ISIS operation.

The hacker group also posted a map, which shows the intensity of the assault on the United Kingdom. The map comes from an online service called Digital Attack Map that charts attacks around the world, Mirror Online reported.

Other countries that came under attack were Saudi Arabia and Canada, said the report, adding that the assaults in these countries did not match the ferocity of the digital bombardment that Britain is still currently facing.

Catalin Cosoi, chief security strategist at Bitdefender, explained to Mirror Online that cyber attackers rely on a weapon called Distributed Denial of Service (DDoS) attack, which involves flooding systems with web traffic until they collapse.

Anonymous is known to use a DDoS system called the Low Orbit Ion Cannon against its enemies, he said.

"It is extremely difficult to estimate who is behind a DDoS attack, as traffic is usually routed from anywhere in the world. This type of attack is usually comprised of computers or Internet-connected mobile devices that are remotely controlled by hackers," said Cosoi.

"This large hacker-controlled network of devices is usually called a botnet and can be used either for sending spam, disseminating malware, or in this case, performing distributed denial of service attacks. We could assume that the incident is somehow related to recent U.K. media news regarding claims that Britain's infrastructure could potentially be vulnerable to cyberattacks."

On Wednesday night, the ISIS reportedly launched an operation to protect a vital propaganda machinery from Anonymous hackers by shifting one of its key websites onto the dark web, where it is relatively safe from the digital blitzkrieg launched in "revenge" for the Paris attacks, a cyber-security expert said as reported by Mail Online.

The Anonymous hacking group, which has temporarily down ISIS' communications systems earlier, declared total war with the terror group to avenge those killed and wounded in Paris on Friday night. Their call for a cyber war was included in a video posted on YouTube on Nov. 14, according to Time.

ISIS operatives responded quickly to the video release, calling the hactivists collective "idiots.''