Police detained Abdul Muktadir, a 19-year-old Muslim from the port city of Chittagong, on charges that he downloaded and spread anti-Islam photos taken from the Facebook account of a Buddhist in the south eastern town of Ramu.
Muktadir saw the photos after the Buddhist ticked "like" on an "Insult Allah" page. A woman was shown standing on Islam's holy book the Koran and a page of the Koran was flushed down a toilet, said local police chief Salim Jahangir.
On the night of September 29, Muktadir "downloaded the photos to a desktop computer and then showed them to local people, triggering the violence against the area's Buddhist community", he said.
Hours later a crowd of Muslims estimated at 25,000 gathered in Ramu in the Cox's Bazaar district to protest. Some later attacked Buddhist temples and houses.
According to police, more than 20 temples were torched or damaged in Cox's Bazaar and in neighbouring districts in what Buddhist leaders described as the worst-ever violence targeting the tiny minority in Bangladesh.
Police said the attack was organised after reports that many attackers had been bussed in, with 300 arrested at the time.
Interior Minister, Mohiuddin Khan Alamgir, blamed radical Islamists for the unrest but did not name any group or parties.
Buddhists, who make up less than one percent of Bangladesh's 153 million mostly Muslim population, are based mainly in south eastern districts near the border with Buddhist-majority Burma.