Zambia's main opposition leader Hakainde Hichilema and his deputy Geoffrey Mwamba are due to be charged with sedition by a court on Thursday, police said, a move his party said was an attempt by the ruling party to silence dissent.
Two months ago, Hichilema's United Party for National Development (UPND) denounced President Edgar Lungu's re-election as fraudulent but attempts to mount a legal challenge have been unsuccessful.
Hichilema and Mwamba were detained on Wednesday accused of holding an assembly without a permit in connection with an impromptu address he gave on Sept 26 to a group of supporters in the central city of Mpongwe, police said.
Sedition in Zambia is defined by conduct inciting people to rebel against the authority of the state, and carries a maximum penalty of seven years in prison.
"This is a scheme by the party in power to harass the opposition and instill fear in them," UPND spokesman Charles Kakoma said.
"They know that they did not genuinely win the last elections and want to instill fear in those who want to speak out against what happened that they risk being arrested."
Hichilema continues to dispute the August election outcome and in a statement before his arrest compared the government's conduct to the limitation of freedoms experienced when Zambia was still a British colony.