Social media accounts of the Tunisian branch of radical feminist group Femen have been hacked by Islamist cyber-pirates, after a radical imam issued a fatwa against one its members for posting a topless picture of herself on Facebook.

Hackers replaced pictures and other material on Femen Tunisia's Twitter and Facebook accounts with religious messages and videos of verses from the Koran.

"Thanks to God we have hacked this immoral page and the best is yet to come," read one message signed by "Al-Angour".

"The page has been hacked and God willing, this debauchery will disappear from Tunisia," read another. The accounts have been suspended.

The attack came after young Tunisian activist Amina, 19, posted on a sister Facebook page (Femen-Tunisian fanpage) a topless picture of herself with the words "F**k your morals" written across her chest.

Radical imam and politician Adel Almi, who heads the Commission for the Promotion of Virtue and Prevention of Vice, said the girl deserved to be whipped or even stoned to death, Tunisian newspaper Assabah reported.

The Ukrainian women's group, that rose to fame with topless demonstrations in favour of women's rights and democracy, answered with a call for Tunisian women to send their topless picture in support of Amina.

"We are furious because of the barbarian threats of the Islamists about the necessity of reprisals against the Tunisian activist Amina," Femen said.

"We are afraid for her life and we call on women to fight for their freedom against religious atrocities.

"Use your body as a poster for the slogans of freedom. Bare breasts against Islamism," Femen said.

Femen Tunisia's Facebook page was hacked after a Tunisian woman named Meriam replied with an image and the message "My body belongs to me!"

Feminists in Tunisia are concerned that women's rights are being neglected in the drafting of a new constitution because of pressures by ruling Islamist party Ennahda, which came to power after the Arab Spring revolution of 2011.

Tunisian activist Amina, 19, posted a topless picture of herself on the Internet (Femen)