Hackers claiming to belong to the Anonymous Internet freedom group posted video messages on Facebook pages of Tunisian Islamists, threatening reprisals over their efforts to introduce Salafist laws.
"We are fighting you... your emails, your bank accounts and transactions will be probed, your hard discs will be copied," said a man wearing the Guy Fawkes mask that has become a trade mark of Anonymous members.
"If the Tunisian government won't stop your activities in the weeks to come, Anonymous will," he added.
"We are not against religion, we are Muslims, but we are defending freedom in our country," a separate written message said, posted along with images of the Tunisian flag.
Tunisia's moderate Islamist leaders, who took power following last year's ouster of strongman Zine El Abidine Ben Ali, are under pressure from a radical Muslim fringe.
The ultra-conservative Salafists have in recent months demanded full-face veils for female university students, castigated a TV channel for an allegedly blasphemous film and beaten up journalists at a protest.
Among the sites hacked was that of the Hizb ut-Tahrir party, which staged an international women's conference near Tunis at the weekend calling for the return of the caliphate, the Islamic system of government which held sway over the Muslim world for hundreds of years.
The attacks came as Tunisia prepared to mark a national day of Internet freedom on Tuesday in memory of blogger Zouhair Yahyaoui, who died on March 13, 2005 at the age of 36 after arrest and torture under Ben Ali's regime.
President Moncef Marzouki is to officially declare the national day and decorate Yahyaoui's family.
"I am glad the government has remembered my son and that his country is paying homage to him at last," his mother Khadija told AFP.
She said her son hardly slept, but sat in front of his computer launching attacks on the corruption and repression of the regime.
His five sisters and two brothers were punished by being forbidden to work, she added.
Founder of an online satirical newspaper, Yahyaoui published an open letter to Ben Ali condemning injustice in Tunsia. Sentenced in 2002 to 20 months in prison, he was tortured and staged three hunger strikes against his treatment.
Released in late 2003, he died of a heart attack.