Police had surrounded Ben Mbarek's home on Wednesday and briefly detained his father, who is also a prominent dissident, on Thursday according to his sister, the lawyer Dahlia Ben Mbarek.
Salsabil Chellali, the Tunisia director at international monitoring group Human Rights Watch, said Saied was going after his critics "with utter abandon."
"The message in these arrests is that if you dare to speak out, the president can have you arrested and publicly denounce you while his henchmen try to build up a file against you based on remarks you made or who you met," he said in a statement.
The police and interior ministry have not made any comment on the wave of arrests this month that has targeted prominent politicians, protest leaders, media figures and others critical of Saied.
Protests against Saied have demanded that he step down and have accused him of a coup for shutting down parliament in 2021, moving to rule by decree and writing a new constitution that he passed last year in a referendum with low turnout.
Saied, who has said his actions were legal and necessary to save Tunisia from chaos, has called his critics traitors and criminals and has said some of those arrested were behind food shortages that economists have blamed on weak state finances.