Aroosa Khan's son was chatting on WhatsApp but suddenly found himself the target of "vigilante" investigators who accused him of having committed blasphemy online, a crime that carries the death penalty in Pakistan.

The 27-year-old is one in hundreds of young men standing trial in Pakistan courts accused of making blasphemous statements online or in WhatsApp groups, an offense for which arrests have exploded in recent years.

Many of the cases are being brought to trial by private "vigilante groups" led by lawyers and supported by volunteers who scour the internet for offenders, rights groups and police say.