It is the latest arrest of Mr Saied's political rivals.
Police this year have detained more than 20 leading political figures, accusing some of plotting against state security. Mr Saied has described those detained as “terrorists, traitors and criminals”.
“Moussi was detained for 48 hours in charges of processing personal data, obstructing the right to work, and assault intended to cause chaos,” lawyer Aroussi Zgir said.
Authorities were not immediately available to comment.
An assistant of Ms Moussi said in a video on social media that she was “kidnapped” in front of the Carthage Palace.
Ms Moussi, who leads the Free Constitutional Party, is a supporter of late president Zine El Abidine Ben Ali who was toppled by mass protests in 2011.
In recent months, the party has organised protests against Mr Saied. Ms Moussi has accused him of ruling outside the law and that she is ready to make personal sacrifices to save Tunisia.
Dozens of angry supporters of Ms Moussi protested in front of the La Goulette police station, shouting slogans against Mr Saied amid a heavy police presence. Police had cordoned off the building.
On Tuesday, Ms Moussi said that she went to the presidential reception office to file an appeal in local elections expected at the end of the year. She said this step was necessary to file an appeal in the Administrative Court at a later date.
Mr Saied, a retired law professor, was elected president in 2019 and shut down the elected parliament in 2021 to rule by decree. Opponents described his actions as a coup. Mr Saied has said he needed to save Tunisia from years of chaos, denying coup accusations.
On Friday, jailed opposition leader Rached Ghannouchi, another critic of Mr Saied, began a three-day hunger strike. Five other prominent opposition figures also went on hunger strike in prison.