Internet services have been suspended and schools shut after four people were killed in violence over the survey of a mosque in Uttar Pradesh’s Sambhal on Sunday.

Chaos erupted in Sambhal Sunday morning when a court-ordered survey of the Mughal-era Shahi Jama Masjid led to violent clashes between the locals and the police. The mosque is at the centre of a contentious legal battle over claims that it was built on the site of a Hindu temple.

The protesters also set some vehicles on fire and threw stones at the police, who used tear gas and batons to disperse the mob.

In the chaos, about 20 police personnel were also injured. A constable, who suffered an injury to the head, is said to be critical.

Twenty-one people, including three women, have been detained and a probe has been launched, an official said, adding that those accused in the violence would be booked under the stringent National Security Act (NSA).