Ivory Coast youth leader Charles Ble Goude, a key ally of deposed strongman Laurent Gbagbo, is under house arrest, a presidential spokesman says.
The BBC's John James says the spokesman confirmed the detention after rumours about Mr Goude's whereabouts
Mr Ble Goude, known for his vitriolic speeches, mobilised thousands of young men to join the army in the final days of the four-month election dispute.
Many other members of his Young Patriots group were given weapons.
He refused to accept Mr Gbagbo had lost November's elections and organised street protests in his support.
Mr Gbagbo was captured on Monday by President Alassane Ouattara's forces after he refused to hand over power.
Our reporter says Mr Ble Goude was not one of those Gbagbo supporters at the presidential residence who were taken into custody.
He was put under UN sanctions in 2006 accused of inciting attacks against UN personnel.
Presidential spokesman Patrick Achi said he was now being held in a secure place.
Mr Ble Goude's aide, Semi Bi, told the BBC afterwards that he did not know about the arrest or where he was.
Most moderate and undeclared members of the Gbagbo government have now pledged allegiance to President Ouattara, our correspondent says.
Others have fled the country, often to neighbouring Ghana, or sought diplomatic protection, he says.