The Yoruba Nation rally was held on Saturday in Lagos despite police warning against gatherings.
The procession at Ojota ended abruptly after operatives pounced on the participants as more joined.
The agitators, called out by Sunday Igboho, a Yoruba secessionist, were not allowed to converge at the venue, Gani Fawehinmi Freedom Park.
The police explained that the reason was to forestall a breakdown of law and order, citing the carnage that occurred during the End SARS protest last October.
On Sunday, the names of persons arrested at Ojota emerged. The 21 advocates of secession include one woman.
Their names: Abiodun Taiwo (female), Adagunodo Babatunde, Oluwafemi Adeleye, Oloye Taiwo, Saheed Kareem, Adebayo Waheed, Akinbode Sunday, Lawal Akeem, Samuel Ire, Ogundile Dare.
Others are Tajudeen Bakare, Abiodun Adenuga, Azeez Adetayo, Rasheed Shittu, Quadri Ishola, Oluwasegun Arire, Kunle Aremo, Olamilekan Lolade, Tunde Lawal, Musbau Rasaq and Bamidele Akomolafe.
The police paraded some caught with charms at Ojota before they were taken away.
Security agencies planned to use rally to apprehend certain separatists on their radar.
It is still unclear if some are in custody. Also yet to be ascertained are the charges the detainees will face in court.
However, the police denied killing a girl found lifeless after she was hit around the chest suspected by a stray bullet.
Public Relations Officer, Muyiwa Adejobi called the allegation a calculated attempt to create tension in the state.
The spokesman, in a statement, insisted that officers did not fire a single live bullet.
He said the corpse found at a location far from Ojota had dried blood stains “suggesting that the corpse is not fresh”.
“After a close look at the corpse, a wound suspected to have been sustained from a sharp object was seen on it”, Adejobi noted.
Also during the rally, a woman accused security agents of recklessness after the discovery of a tear gas canister at her residence.
Apart from Ojota, Lagos was generally peaceful. Nigerian forces were deployed to several areas considered volatile.