Elo Ayeto, coordinator of Somos+ Sociedad Civil group, was detained on Saturday, international human rights day, "for having organised a conference on human rights in Equatorial Guinea without official authorisation" lawyer Angel Obama Obiang Eseng said.
"The motive for his arrest and detention ... does not hold up," he added.
"He organised a conference in a private establishment" and not a public place for which authorisation would be needed.
The authorities made no comment when asked about the arrest.
A year ago Elo Ayeto was thrown out of the sole political party that Malabo has not banned.
He was arrested in 2019 for attempted assassination of President Obiang, before the charges were reduced to defamation and threats against the president.
He was released without trial after a year in detention.
On September 25, Anacleto Micha Nlang, co-founder of banned rights group "Guinea is also ours", was arrested. He remains in detention.
The small, Spanish-speaking central African country is one of the most closed and authoritarian in the world and has been led by President Teodoro Obiang Nguema Mbasogo for more than 43 years.
President Obiang was re-elected on November 20 with 94.9 percent of the votes.
The run-up to the election saw a crackdown that state media justified as a bid to counter a "foiled plot" by the opposition to attack embassies, petrol stations and the homes of ministers.