The State Security Service (SSS) on Sunday confirmed that its operatives took Omoyele Sowore into custody.
Mr Sowore, a social critic and publisher of Sahara Reporters, was violently arrested in Lagos at midnight on Saturday.
He was taken into custody because he called for a revolution in Nigeria, SSS spokesperson, Peter Afunanya, told reporters in Abuja Sunday afternoon.
Mr Afunanya was unable to say whether or not the SSS has any credible intelligence that confirmed Mr Sowore has the capacity to execute a takeover of government in Nigeria.
The spokesperson, however, said the secret police was "aware" that Mr Sowore had been in touch with foreign actors to destabilise Nigeria.
The comments marked the first time the SSS will brief Nigerians since Mr Sowore was arrested.
The police have already deemed the planned protest an act of terrorism and threatened a clampdown on participants.
Mr Sowore and other organisers of 'RevolutionNow' movement have condemned federal authorities for lying against them that the protest, scheduled to begin on August 5, would be violent.
They have vowed to carry on with the demonstration across 21 Nigerian cities despite the arrest of its leader.
The SSS declined to say where Mr Sowore was being kept and when he would be arraigned in court.