At least four people were killed and nearly 50 injured on Sunday after police fired on thousands of Bangladeshi Muslims protesting a Facebook post by a Hindu who allegedly defamed Prophet Muhammad (PBUH), officials said.Mob attacks over Facebook posts perceived to be blasphemous have emerged as a major headache for security forces in Bangladesh, where Muslims make up some 90 per cent of the country's 168 million people.Some 20,000 Muslims demonstrated at a prayer ground in Borhanuddin town on the country's largest island of Bhola to call for the execution of the young Hindu man, who was arrested Saturday over charges of inciting religious tension.Police said they opened fire in self-defence after some of the crowd threw rocks at their officers."At least four people were killed and up to 50 people were injured," police inspector Salahuddin Mia told AFP.He said extra police and border guards were being deployed in the town.The death toll is expected to rise, with Bhola Sadar Hospital duty doctor Tayebur Rahman telling AFP at least seven of the 43 people taken to hospital were fighting for their lives."We have sent the critically injured people to the regional hospital in Barisal (city). Their conditions are not stable," he said.In 2016, angry Muslims attacked Hindu temples in an eastern town over a Facebook post was put up allegedly mocking one of Islam's holiest sites.In 2012, Muslim mobs torched Buddhist monasteries, houses and shops in the coastal Cox's Bazar district following a Buddhist youth's alleged defamatory photo post of the Holy Quran.Bangladesh has also experienced a number of attacks on people from religious minorities, secular bloggers, publishers, writers and foreigners, many claimed by militants.