Assistant Superintendent of Police Ibrahim Samura said on state radio Saturday night that Charles Francis Margai was detained for alleged subversion.
Margai earlier in the week spoke about a land dispklute he has with Sierra Leone's first lady, Sia Koroma. Margai said he would fight for his land and said he would be backed by militia forces loyal to him.
The threat of militia violence comes as Sierra Leone is recovering from a 10-year civil war that ended in 2002.
Margai was among the candidates who contested the presidential election in 2012 election and lost to Ernest Bai Koroma. Margai's party does not hold a seat in parliament.