Plainclothes officers in Tunisia's capital on Friday arrested Noureddine Bhiri, a senior official of the Ennahdha party, Agence France Presse reported.
The movement condemned the arrest of Bhiri, a former justice minister and deputy president of the Islamist-inspired Ennahdha, as a "dangerous precedent".
Bhiri is close to Tunisia's parliament speaker Rached Ghannouchi, head of Ennahdha.
"Plainclothes agents in two cars arrested Noureddine Bhiri when he was leaving home with his wife," Samir Dilou, a lawyer and former legislator, told AFP.
In a statement issued late Friday, the interior ministry said it had ordered two people, whom it did not identify, to be placed under house arrest.
This "preventive measure was dictated by the need to safeguard national security," it said, without elaborating.
President Kais Saied on July 25 sacked the Ennahdha-supported government and suspended parliament.
He later took steps to rule by decree, and in early December vowed to press on with reforms to the political system.
The former law professor announced an 11-week "popular consultation" to produce "draft constitutional and other reforms" ahead of a referendum set for July 25.
His opponents have denounced a "coup" and warned against what they see as Saied's wish to settle scores with those he has called "enemies" but never names.
A Tunisian court last week sentenced in absentia exiled former president Moncef Marzouki, a fierce critic of Saied, to four years in prison.
Bhiri was "arrested brutally and taken to an unknown destination", Dilou said.
Agents also seized the mobile phone of Bhiri's wife Saida Akremi, who is a lawyer, he added.
In a statement, Ennahdha said that Bhiri was being questioned by authorities and denounced "a kidnapping and dangerous precedent marking the country's entry into a tunnel to dictatorship."
Party official Mohamed Goumani told a press conference that Saied and the interior minister "bear responsibility" and decried the "intimidation" of those opposed to the president's moves.
