The 21 social activists were taken into preventive custody here for trying to take out an "unauthorised" procession, a senior police official said.
The procession was planned against the backdrop of a controversy over a book written by writer Kancha Ilaih.
The book, 'Samajika smugglers komatollu' (Komatis are social smugglers) triggered a row with those belonging to the community taking objection to it.
Meanwhile, CPI(M) state secretary Tammineni Veerabhadram claimed in a release that the procession was planned to express solidarity with the associations of Dalits and others.