Police said on Monday that an explosion that killed seven people and injured 32 in a packed cinema hall in Ludhiana was a "terrorist" bombing.
Hundreds of people -- mainly poor migrant workers -- were crammed into the theatre in the industrial city of Ludhiana in Punjab state to watch Sunday's late-night screening of a new Bollywood comedy.
"It was a bomb blast. It is a terror act. We are trying to find out the exact nature of explosives used," said a senior police official on condition of anonymity.
Police have not named any suspect but a series of blasts across India in recent years have been blamed on Muslim extremists allegedly linked to Pakistan and Bangladesh.
India's 140 million Muslims celebrated Eid al-Fitr on Sunday, marking the end of the fasting month of Ramadan, and one of the biggest Islamic festivals.
Several of the wounded were fighting for their lives.
"These are multiple injuries... Most of them are serious injuries," a doctor told NDTV news network.
Television footage showed the floor of the cinema strewn with shards of broken glass and stained with blood. Shoes and pieces of torn clothing also littered the devastated site.
The Home Ministry in New Delhi said it was still "too early" to draw any conclusions. Besides Islamic militancy, the Punjab was also the scene of a bloody Sikh insurgency in the 1980s.
Security at bus and railway stations and important buildings across the state was tightened, a Punjab police spokesman said.
The latest blast came after two people were killed and nearly a dozen wounded on Thursday in a bomb blast at one of India's most revered Islamic shrines in the northern state of Rajasthan.
India sounded a nationwide alert after that attack as the country prepares for the main Hindu festival season starting on October 21 with Dussehra which marks the triumph of good over evil.