The attack was carried out last night after the activists had lodged a police complaint against a man, accusing him of indulging in conversion, Udupi Superintendent of Police Boralingaiah said.
A group of Bajrang Dal activists barged into the man's residence at Moodubelle, also a prayer house, and assaulted the people gathered there, he said, adding, 19 of them had been arrested.
The coastal districts of Udupi and Mangalore had in the past seen attacks on churches triggered by suspicion that they were carrying out conversion.