We examine the competing processes of ethnicization taking place among Hindu and Muslim religious communities in the coastal region of Karnataka state in South India in the broader context of hegemonic ascendancy of Hindu nationalism in the country. We describe how an array of militant Hindutva organizations use an institutionalized system of religious vigilantism and violence against minorities to construct an ethnicized, exclusivist moral community of Hindus in the region. This construction of an ethno-Hinduism by a clever depoliticizing of caste inequalities and violence seeks to produce and naturalize religious difference into an incorrigible and exclusivist ethnic identity that thrives on a continuous process of enemy-making. Responding to this predatory ethnicization executed by militant Hindutva organizations, and capitalizing on the pervasive sense of alienation and anxiety of the Muslims in the region, radical Islamic organizations engage in a counter-predatory ethnicization of Muslim communities in the region. These organizations, while officially articulating secular positions, use the language of self-defence and securitization, coupled with radical Islamic identity for mass mobilization, to create an exclusive Islamic moral community, often mirroring the tactics of Hindutva vigilante organizations. We conclude that these competing processes of ethnicization of religious identities and the emergence of a ‘vigilante public’ will have far-reaching consequences for the central facets of democracy such as citizenship and secularism while leaving the socio-cultural spaces in coastal Karnataka highly polarized on religious lines and peaceful co-existence of religious communities challenging and undesirable. © The Author(s) 2020.