(Ahlul Bayt News Agency) - About 2,000 protesters, including 300 monks, shouted slogans in the central town of Dambulla. They tore down a signboard before police intervened and stopped them.
A mosque official told the BBC he and several dozen companions were trapped inside and feared the crowd would destroy the building.
Overnight the mosque had been targeted by a fire-bombing - no-one was hurt.
The BBC's Charles Haviland in Colombo says the tensions have been growing in the neighbourhood.
Shortly after the protest the mosque was evacuated and its Friday prayers cancelled.
One monk, Akmeemana Dayaratna, told The Associated Press that the mosque and a Hindu temple are illegal constructions inside a Buddhist zone and must be demolished.
Mohammed Mustafa, a 39-year-old Muslim worshipper, said that the mosque has been there for decades and is legal.
Buddhism is the religion of the majority of the population in Sri Lanka.
About 7 percent of Sri Lanka's 20 million people are Muslims. About 74 percent are Sinhalese, who are mostly Buddhists, while about 18 percent are Tamils, who are predominantly Hindus or Christians.
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