Riahi has neither been released from custody in Gorjani police station, nor granted access to her lawyer, Laila Ben Debba, who has spoken to her only for a few minutes, Al Jazeera has learned.
Ben Debba told Al Jazeera: "On November 3, the police phoned me and asked for Fatma to go back to them for more interrogation. Fatma has been held there up until this day.
"The accusations she now faces are about reports published on her blog but not by herself.
"On November 3, she was re-summoned and asked to bring her 'publishing apparatus', and this apparatus was in her home, and her home key was with me, she came to my house accompanied by three policemen to take the key. She talked to me and affirmed that she was not treated in any offending way.
"I went to the unit to know where Fatwa was, they said she was in custody and that there were no official charges facing her as her file had not yet been referred to the prosecution in Tunisia."
If prosecuted, Riahi could face criminal defamation charges that potentially carry a prison term of to up to three years.
"Since her arrest, Fatma's rights have been violated. The Tunisian authorities are using the pretext of her arrest in an attempt to discover the identity of anonymous Tunisian bloggers"
Reporters Without Borders statement
The uthorities are investigating whether Riahi is hiding behind the pen-name of Blog de Z, a controversial Tunisian cartoonist blogger whose political satire has enraged the government.
Blog de Z's most recent post was published three days after Riahi was first taken into custody.
Reporters Without Borders (RSF) said on Friday: "Since her arrest, Fatma's rights have been violated. The Tunisian authorities are using the pretext of her arrest in an attempt to discover the identity of anonymous Tunisian bloggers.
"We demand the authorities drop the charges against Fatma Arabicca and release her immediately."
Riaihi deleted her blog three days before her arrest and her Facebook account has been removed.
'Free Arabicca' campaign
Lina Ben Mhenni, a friend of Riahi, told the Los Angeles Times in an email on Friday: "We talked about the Criminal Brigade [summoning] her, her worries, but we were optimistic as we know that she didn't do something wrong."
A Free Arabicca campaign blog has been launched by fellow Tunisian bloggers in support for Riahi, as well as a facebook page.
Global Voices, a blogger advocacy website, ranks Tunisia just behind Iran as one of the most repressive countries towards bloggers and online activists.
Zouhair Yahyaoui, one of Tunisia's most famous imprisoned former bloggers, was arrested in 2000 after inviting readers to vote on whether Tunisia was a "republic, a kingdom, a zoo or a prison".
Yahyaoui died of a heart attack in 2003 after reportedly being severely tortured.
In another development, France voiced concern on Friday over the fate of a Tunisian journalist and vocal critic of Zine El Abidine Ben Ali, the Tunisian president, who was arrested last week for allegedly assaulting a woman and faces trial.
Taoufik Ben Brik was detained on October 29 and is being held in a town outside Tunis pending trial later this month.
RSF said in a statement last week that the charges were bogus and that Ben Brik was too ill to stand trial.