Homepages of several French websites have been replaced by phrases like "There is no God but Allah", "Death to France" or Death to Charlie" written against a black background, often with the signature #OpFrance. The technique termed "defacement" consists of taking control of an Internet site and modifying its content.
Since last week's attacks in Paris that killed 17 people, various sites - mainly those of city halls, schools, universities, churches and businesses - have been hacked by groups claiming to be Islamists from North Africa or Mauritania.
Users were directed to a site broadcasting a fundamentalist speech when they logged onto the official website of the Lot department in south-west France. A Tunisian group calling themselves "Fallaga Team" claimed to behind this incident.
A website showing a message in Arabic and another in French which said "I confirm that there is only one God and that is Allah. I confirm that Mohammed is the messenger of Allah" replaced the home page of the Caen Memorial, a museum and war memorial in the northern town of Caen.
"We are facing groups of activists who are forming and dissolving very quickly," said Gerome Billois, a cyber-security expert at Solucom.
"I can't remember having seen such a big hacking campaign in such a short period of time," he said, adding that "several hundreds of sites had been affected."
"We can speak of cyber-jihad, and hacking is just the tip of the iceberg and also the least dangerous since the only consequence is the display of an ideology," said Thierry Karsenti, European technical director at cyber-security firm Checkpoint.
He added that it was technically fairly easy to take control of these sites given that they were either poorly protected or not updated regularly enough.
The same hacking techniques are being used by the opposing camp. After calls from the hacker group Anonymous to avenge Charlie Hebdo, activists claimed to have attacked jihadist propoganda sites.
"Wait for a huge reaction from us," the group said after launching a Twitter account called @OpCharlieHebdo following the Paris attacks.
The message on the social network said that the group, part of Anonymous, was fighting to "defend the freedom of expression and opinion".