Sudan People’s Liberation Movement-in-Opposition (SPLM-IO) spokesman James Gatdet Dak, who was arrested at his Nairobi residence on Wednesday, is being held at the Jomo Kenyatta International Airport Police Station.
Mr Dak is likely to be deported any time from now.
Plainclothes police officers picked him up from his Lavington estate house in Nairobi on Wednesday at around 4 p.m.
“They claimed to be Kenyan national security agents. When James asked them the reason behind his summons, they replied they were going to discuss it in their offices,” Lam Jock, a representative of the Riek Machar-led SPLM-IO in Kenya, told Nation.co.ke after the arrest.
He said the officers refused to allow an SPLM-IO official to accompany Mr Dak.
The arrest came weeks after Kenyan MPs threatened to push for the freezing of assets that Dr Machar and South Sudan President Salva Kiir own in Nairobi in a bid to force them to restore peace in Juba.
Kenya on Wednesday also announced it would withdraw its troops from South Sudan, a day after UN Secretary-General Ban Ki-moon sacked Lt-Gen Johnson Ondieki as commander of the UN Mission in South Sudan.
The announcement follows an official UN report that said he had failed to protect civilians during the violence in Juba in July.