"We ask the government to identify those responsible and investigate their terrorist links," Dilantha Withanage, a top BBS official, said.
"No one knows who they are. They were carrying leaflets which carried words of a distorted national anthem," he said.
The group named "Buddhists Questioning Bodu Bala Sena (BQBBS)" had staged a candle light vigil opposite the BBS headquarters last night leading to a clash with BBS members.
"They were English speakers and their candle light vigil is not something you associate with Buddhists," Withanage said.
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Bodu Bala Sena is leading a campaign which they say targets the growing Muslim extremism in the country.
Less than 10 per cent of Sri Lanka's population of 20 million are Muslim. The majority are Sinhalese Buddhist, while most Tamils are Hindu.
The campaign prompted the main Muslim party and government ally, Sri Lanka Muslim Congress, to call for urgent government action to ensure the safety of the Muslim minority.