Olisa Agbokoba, president of the Civil Liberties Organisation, was picked up at his law office by three state security agents, according to a statement from the CLO.
Abdul Oroh, executive director of the CLO, said his organisation "will not be daunted by this act of intimidation in our quest to ensure a free society governed by law, reason, and social justice."
Mr Agbokoba's arrest came as the head of the police issued a stern warning to pro-democracy activists who may be planning demonstrations on the anniversary next Monday of the demonstration that led to the arrest of Moshood Abiola.
Scores of people have been arrested since Nigeria's military leader, General Sani Abacha, refused to accept Mr Abiola as the winner of the 1993 presidential election. Mr Abacha took power and broke his promise to return the country to civilian rule.
The military government also warned oil workers against disruptions. The oil industry was brought to a standstill last year when workers joined in the campaign for new elections.Reuse content