Paula Yacoubian, a former journalist who had interviewed Saad Hariri in Riyadh last November after he made a shock resignation, made the remarks during an interview with Lebanese Al-Jadid TV on Sunday.
"It seems [Hariri] was threatened but I don't know to what extent… without evidence or documentation I can say that he was detained," Yacoubian said.
"It was clear that Hariri was tired and under pressure. It was clear from the interview that he didn't want to resign."
Before running and winning a seat in the Lebanese legislative elections in May, ex-TV host Yacoubian was the first to interview the premier after he announced his resignation in a televised speech from Saudi Arabia, stunning his country and creating a mystery that has lasted until today.
She initially denied that Hariri had been held against his will and had been intimidated into quitting his post.
During the interview, Yacoubian also touched on the disappearance and alleged killing of Saudi journalist Jamal Khashoggi.
"I hope that Khashoggi is being detained and hasn't been killed but things are pointing towards murder. If this is confirmed, thank God for Hariri coming out [of Saudi Arabia] safe," she said, according to local media.
Khashoggi, who has been critical of Saudi Crown Prince Mohammed bin Salman, vanished after an appointment with Saudi officials on Tuesday at the consulate in Istanbul.
Khashoggi has lived in self-imposed exile in the United States since last year to avoid possible arrest, after criticising Prince Mohammed's intervention in Yemen and the blockade of Qatar.
Saudi Arabia has been engulfed by several controversies since Prince Mohammed was named crown prince and heir to the throne in June 2017.
The young prince has stepped up repression of religious, intellectual and feminist dissidents and has been behind an increasingly aggressive foreign policy.