Three men have been held over the killing of an Indian police officer in mob violence that erupted after rumours spread that cows had been slaughtered.
Subodh Kumar Singh was killed as police clashed with right wing Hindu groups who said police failed to halt the killing of cows in Uttar Pradesh state, reports BBC.
Cows are considered holy by India's majority Hindu population and many states have now banned cow slaughter.
The issue has sparked violence before, but an attack on police is unheard of.
So-called cow vigilantism has been on the rise and led to several killings in the past few years.
Police are now looking for 24 others suspected to be involved in Monday's murder in Bulandshahr district.
Many states, including Uttar Pradesh, have actively started enforcing bans on cow slaughter after the Hindu nationalist Bharatiya Janata Party (BJP) formed India's federal government in 2014.
In Monday's incident, a mob attacked policemen and set fire to a police station in Bulandshahr alleging cow slaughter in the area.
An 18-year-old protester was killed and another policeman was injured in the violence.
The police have registered cases of rioting, attempt to murder and destruction of property against 50 unnamed people.
Shops and schools in the area are shut and more than 1,000 policemen have been deployed, says BBC Hindi's Nitin Srivastava in Bulandshahr.
Our correspondent says incidents of violence over alleged cow slaughter are not uncommon in this region, but a direct attack on the police is unprecedented.
The incident has prompted outrage on social media, with many calling the attack "senseless" and "barbaric".
Local villagers alleged that they found carcasses of some cows near Mahav village on Monday morning. They claimed that the animals had been slaughtered.
"The local police got information about cow slaughter after which they immediately started investigating the matter," Anuj Jha, a top district official, told reporters on Monday.
"But soon after, people in the area blocked the streets and started pelting the police with stones."
Mr Jha added that police officer Subodh Kumar Singh died in the clashes that followed.
It is unclear where the alleged cow slaughter occurred.
Police said they were investigating the incident to find out who instigated the violence.
