In a brief ceremony that included a prayer session at Periyapalayathu Amman Temple in Pattalam, eight Christians and two Muslims were reconverted, Bharat Hindu Munnani-Tamil Nadu said.
They embraced Hinduism and adopted their old names.
"Continuance of personal issues" even in new religion was among the reasons cited by them for returning to Hinduism, Central Chennai District President of the outfit K Satish, who organised the event, told PTI.
Asked how they identified those who wanted to reconvert, he said they held a campaign.
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"We did not persuade or compel anyone," he claimed.
State President of the outfit, R D Prabhu alleged police barred entry (of the activists) into at least four temples in Pattalam area to "scuttle the re-conversion event".
"I was detained at my office in Choolai Thattankulam and was not allowed to move out till evening," he said.
Police said he was detained to maintain law and order.
Asked on the reconversion event, Otteri police Inspector M G Sundar, under whose jurisdiction Pattalam falls, said he had no idea of any "public function" to reconvert.