A Tanzanian court charged rights activist Tito Magoti with money laundering and other offences after holding him in custody for four days, the organisation for which he works said on Tuesday.
Prosecutors in Kisuto court said Magoti and co-worker Theodory Giyan had been charged with leading organised crime, money laundering and possessing computer programmes designed for the purpose of committing crimes.
Both were being held in custody to await a January 7 hearing.
Magoti, of the Legal and Human Rights Centre (LHRC), was arrested by plain-clothed officers who bundled him into an unmarked car on Friday, initially raising fears that he had been abducted.
Although Dar es Salaam regional police commander Lazaro Mambosasa confirmed his arrest, family and friends were unable to contact him until Tuesday's court appearance.
A statement from the LHRC said the suspects were in good health but that Magoti appeared very tired.
They had been questioned about their relationship with fellow activists Maria Sarungi and Fatma Karume and with opposition politician Zitto Kabwe, the statement added.
"We are closely following the case and will provide legal representation," said the LRHC.
Amnesty International and Human Rights Watch (HRW) both denounced what they said was the deteriorating human rights situation in Tanzania in statements in late October.
They argued that since President John Magufuli came to power at the end of 2015, his administration has stepped up repressive action against the country's news media, civil society and the opposition.