Amnesty International has called for the immediate release of a young Nigerian it said was arrested after posting a tweet deemed offensive towards the nation's first lady Aisha Buhari.
Illegal arrests are not uncommon in Africa's most populous country, despite having a stable democracy since 1999 after decades of military dictatorships.
Twenty-three-year-old Aminu Adamu Muhammed was arrested on 8 November 2022 at midnight "by security operatives suspected to be DSS operatives, over a tweet deemed to be demeaning to the person of the first lady of Nigeria," the rights group said on Sunday.
"His family and friends alleged that he was held incommunicado and subjected to severe beating, torture and other forms of ill-treatment," Amnesty said.
The Department of State Services (DSS) told AFP on Monday it was not behind the alleged detention, while Nigeria's federal police did not respond to a request for comment.
"Since his arrest, neither his family nor his lawyers have had access to him," according to Amnesty, which suspects he is being held in an "unknown location" in Abuja.
Local media reported that Muhammad commented on a picture of the first lady, tweeting in Nigeria's Hausa language that she had put on weight after "eating poor people's money".
President Muhammadu Buhari, a former army general who was elected as a civilian in 2015 and 2019, is stepping down next year after his two terms allowed by the constitution.
Voted into office on a promise to crack down on corruption, Buhari leaves mounting problems to his successor, from rampant insecurity to an economy in shambles.
Mass youth-led protests over police brutality and bad governance were violently suppressed under his watch.
Twitter was also suspended in Nigeria for seven months last year.