The activists are Nejib Chebbi, Reda Belhaj and Ayachi Hammami. Chebbi is the leader of the opposition Salvation Front and has organized regular protests against Saied for months.
Hammami said a judge opened an investigation at the request of the justice minister on charges of spreading rumors and insulting the president. Last week, Hammami accused Tunisian officials of fabricating terrorist cases against judges dismissed by Saied last year.
Chebbi said that he would not respond to any lawsuit because the case is politically motivated.
"It is a farce ... it is fabricated political file," Chebbi said.
Recently, Saied, 64, had warned that judicial actions must be taken against those who defame the "symbols of the state," and that such actions should be considered as an attack on state security.
The justice ministry was not immediately available for comment.
Saied seized control of executive powers in July 2021 and later dissolved parliament. His critics and opponents have described the move as a coup. Saied's opponents accuse him of imposing a dictatorship and strengthening one-man rule.
The Tunisian General Labor Union (UGTT), the country's powerful labor union has also criticized the president, saying that it rejects his autocratic approach and will not remain silent.
Last week, the union with more than a million members announced its intention to "occupy the streets" soon to show its disapproval of the government's 2023 austerity budget. It's the strongest challenge to the government of President Kais Saied yet.