By Geetinder Garewal

CHANDIGARH, India, Oct 15 (Reuters) - Indian police say they suspect Muslim militants allied with Punjab separatists were behind a blast that killed six people and injured 32 at a crowded cinema hall in northern India.

The multiplex Shingar Cinema in the industrial city of Ludhiana in Punjab state was packed with about 600 people at the time of the blast on Sunday. Police say plastic explosives were used in the bomb.

"The leads into the blast indicated Muslim fundamentalists could have collaborated with Babbar Khalsa International and Khalistan Commando Force (both Punjabi militant groups) to plant the bomb," senior police intelligence officer Jagdish Mittal told reporters in Ludhiana.

Security forces have increased vigil in areas like multiplexes, and railway and bus stations since the attack.

The Indian province of Punjab saw over a decade of violence in the 1980s when Sikh militants demanded a separate Sikh homeland they called Khalistan.

Indian intelligence agencies also say Islamist militant groups, backed by Pakistani spy agency ISI, increasingly target Muslim and Hindu religious places to trigger communal clashes between the two communities.

Sunday's blast followed a bomb explosion last week after evening prayers at the Muslim Ajmer Sharif shrine in northwestern India, in which at least two people were killed.

Newspaper reports say police suspect Bangaldeshi militants were involved in that blast, again backed by Pakistani militants and the ISI.

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