Zambian musician and activist Chama "Pilato" Fumba was arrested Saturday for addressing an unauthorised rally in the southern city of Livingstone, police said.
Police detained Fumba and another man as he attended a meeting on government corruption, said witnesses.
"Chama Fumba and others were having a meeting with members of the public at a centre for Roman Catholic church without notifying the police," Esther Katongo, a spokeswoman for the service, said in a statement.
"Officers went to the centre and picked up Fumba who has been detained and arrested for the offence of unlawful assembly."
If convicted, he faces up to five years in jail.
Hakainde Hichilema, a leading figure in the opposition United Party for National Development (UPND) called for Fumba's "immediate and unconditional release", in a Facebook post.
The police should "cease abusing the POA (Public Order Act) to advance (the) interests of the PF", he added, referring to the ruling Patriotic Front party.
This is not the first time the outspoken hip-hop artist has been arrested.
In 2017, he was detained with several others as they picketed parliament over how a lucrative contract to acquire fire engines had been handled. They questioned the transparency of the procurement process.
Fumba was also arrested in May last year following an uproar over his hit song, "Koswe Mumpoto" (Rat in the Pot). Supporters of the Patriotic Front saw it as an attack on President Edgar Lungu and his government.
Right campaigners such as Amnesty International accuse Lungu of stifling the opposition and activists.
The country's opposition has also accused Lungu of increasingly authoritarian behaviour.