Former justice minister Ruben Maye Nsue Mangue, 59, was arrested on the night of August 7 in the eastern town of Mongomo, after being summoned there to a meeting with the ruling party, one of his brothers, Ruben Monsuy, told AFP.
After the meeting, "Ruben was arrested and taking to an unknown location after refusing to apologize to the head of state," he said.
"We have no idea of his whereabouts."
The ministries of information and justice did not immediately reply to a request for comment from AFP.
Five days after the arrest of the ex-minister, who is also a clergyman, the justice ministry issued an order forbidding him from preaching and accusing him of "provoking public disorder".
On July 25, the former justice minister in a Whatsapp voice note that went viral criticized the president's management of the country, describing him as a "demon... holding his people prisoner", and calling for national dialogue.
A coalition of exiled opposition parties, civil society groups and human rights monitors confirmed the arrest.
Ruben Maye was justice minister from 1998 to 2004, then named ambassador to the United States in 2013.
Guillermo Nguema Ela, a Europe-based coordinator in the coalition, told AFP via telephone that the former minister had been detained "without an arrest warrant or appearing in court without 72 hours" of his detention as required by law.
Opposition politician Joaquin Elo Ayeto -- who was himself held for almost a year over "defaming" the president until his release in early 2020 -- said civil society was worried.
"We are sounding the alarm bell," he said.
President Teodoro Obiang Nguema Mbasogo, 80, has ruled Equatorial Guinea with an iron fist for more than 43 years, making him the longest-serving non-monarchic leader in the world.
International rights groups regularly accuse the authorities in the former Spanish colony of human rights abuses, including of arbitrary detentions, forced disappearances and torture.
