Rights groups say a prominent human rights lawyer in Equatorial Guinea went missing two days ago during a visit to a prison, raising concerns he may be unlawfully detained.

Human Rights Watch and EG Justice on Wednesday called on the government to investigate Fabian Nsue Nguema's disappearance. The groups say Nguema is active with the opposition party and has been harassed by the government before.

Human Rights Watch says Nguema went to Black Beach prison on Oct. 22 to meet with a client and did not return home. It says one family member reported they saw him in detention there.

Equatorial Guinean law prohibits secret and warrantless detentions, and stipulates that charges must be filed within 72 hours of an arrest. Authorities have violated these laws in recent months.