At least three people were killed and more than a dozen injured in a grenade attack at Nirankari Bhawan gurdwara in Amritsar, Indian media reported on Sunday.

Around 250 followers of the Nirankari spiritual group, who are considered heretics by most Sikhs, had gathered for morning prayers in Amritsar's Adliwal village when two men reportedly arrived on a motorcycle and threw the explosive at them.

“Three people have died and 15-20 are injured,” senior police officer Surinder Pal Singh Parmar told reporters.

Nirankari followers are at odds with mainstream Sikhs who dominate in Punjab. Unlike most Sikhs, Nirankaris accept the authority of a living guru (spiritual guide). Its members also differ from other Sikhs in their disapproval of the militant brotherhood of the Khalsa.

Eyewitnesses said that the attackers also had assault rifles, The Hindustan Times reported. A witness said the attackers were wearing white and had covered themselves in shawls.

Police officer Dinesh Singh says the two masked men threw the grenade at a prayer hall. It exploded away from the main congregation in the compound, where hundreds of devotees were praying, he said.

The injured are being treated in the Guru Nanak Dev Hospital and Ivy Hospital in Amritsar. Authorities have not yet blamed anyone for the attack.

Indian province of Punjab has been largely peaceful for over two decades after the state's authorities brutally suppressed a violent insurgency for an independent Sikh homeland in the 1980s and early 1990s.

The attack has triggered a security alert across the state as it gears up for the big Guru Nanak Prakash Diwas festival on Friday.