Santiago Ebee Ela, 41, died at the central police station in capital Malabo on Saturday night as a result of "cruel torture", Citizens for Innovation (CI) said in a statement.
"The death of Santiago Ebee Ela is a consequence of the cruel and inhumane treatment shown to CI detainees by security forces of the PDGE regime," it said, referring to the ruling Democratic Party of Equatorial Guinea headed by President Teodoro Obiang.
The death was not confirmed by Equatorial Guinean authorities nor reported by state media.
Ebee Ela was arrested at home on the night of January 2, according to CI, which believes more than 200 of its activists have been detained over the past two months as part of a government crackdown following the mid-November elections.
During the vote, the PDGE won 99 out of 100 seats in parliament in a result condemned as fraudulent by the opposition.
The sole opposition MP, who is from CI, did not attend the opening of parliament on Friday as he was also in detention, the party said.
Obiang, who has ruled the oil-rich West African nation since 1979, is Africa's longest-serving president.
