Pakistan's human rights body has strongly condemned the arrest of an 11-year-old Christian girl with Down syndrome after she was accused of "blasphemy". Tensions have caused around 600 Christians to flee the area.

The girl, identified as Rifta Masih from the rural area of Mehrabadi in Islamabad, was accused of burning a Noorani Qaida, a booklet used to learn the basics of the Holy Koran, last Thursday. It was also alleged that she had thrown the booklet in the garbage after putting it in a plastic bag.

Following the accusation, which appears to be untrue, a mob demanded the arrest of the girl and threatened to burn houses belonging to Christians.

"The fact that the girl is a juvenile and suffers from Down syndrome only makes the charge more preposterous and barbaric," the Human Rights Commission of Pakistan said in a statement. "It is also extremely disturbing to note that the police allowed a mob to surround the police station and demand that she be handed over."

The commission has asked the authorities to "immediately release" the girl and protect her family and the "frightened Christian community" in the area.

According to Paul Bhatti, Pakistan's minister for National Harmony and a Christian, the girl was known to have a mental disorder and that it seemed "unlikely she purposefully desecrated the Koran". "From the reports I have seen, she was found carrying a waste bag which also had pages of the Koran," he told the BBC.

"This infuriated some local people and a large crowd gathered to demand action against her," Bhatti said. "The police were initially reluctant to arrest her, but they came under a lot of pressure from a very large crowd, who were threatening to burn down Christian homes."

Bhatti also said more than 600 people had fled the Christian neighbourhood out of fear.

Bhatti's brother, Shahbaz Bhatti, was assassinated in March last year for speaking out against the notorious blasphemy law. He was a Christian and minister of Minority Affairs. Shahbaz Bhatti was killed after Punjab's Governor Salman Taseer was assassinated for his advocacy for Pakistani Christian woman Asia Bibi who had been convicted by a trial court for blasphemy.

The blasphemy law, embedded in Sections 295 and 298 of the Pakistan Penal Code, is frequently misused to target religious minorities – Christians, Shi'as, Ahmadiyyas and Hindus – and allows Islamists to justify killings. Extremist Islamists believe that killing a "blasphemous" person earns a heavenly reward.

A mere accusation is enough to have a person arrested. There is no provision in the law to punish a false accuser or a false witness of blasphemy. Some local Muslims seek revenge by making an allegation against his or her adversary who is a non-Muslim. Many who are accused of blasphemy are killed by mobs extra-judicially.