The general has been accused of discussing plans to remove Sassou Nguesso from power with a French intelligence agent in a video filmed in 2007.
"He has been detained for questioning at the DGST (security services),"Mokoko's lawyer Yvon Eric Ibouanga aold AFP yesterday.
He added that the investigation against Mokoko, who was military chief of the oil-rich African nation from 1987 to 1993, was still at the "preliminary" stage.
Witnesses said Mokoko was escorted to DGST offices by several police officers.
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The general took less than 14 per cent in the March 20 election, which returned Sassou Nguesso to the presidency with more than 60 per cent of the vote.
The opposition charged that the poll was marked by "massive fraud".
Mokoko had already been questioned over the video on the eve of the election.
He has been living under surveillance at his home in central Brazzaville since the vote.