Doctors say at least four of the injured are in a critical condition

The grenade exploded in a cinema in the town of Gauribari, 270km west of Assam's state capital, Guwahati.

Police have blamed the attack on separatist rebels who treat the 15 August anniversary as a "black day".

Security forces are on alert throughout India to prevent rebel violence during the annual independence celebration.

Transport restrictions

Army and police units are on the look-out for attacks in India's troubled north-eastern states and in Kashmir.

A security cordon has been imposed around the capital, Delhi, where Prime Minister Manmohan Singh is to address the nation on Sunday.

A bullet-proof glass enclosure has been installed around the podium where he will give his speech, in the 17th-century Red Fort complex, police sources said.

A no-fly zone will be enforced over the capital and rail and road transport will be restricted during the celebrations.

Boycott

The attack on the cinema in Assam injured several people, four of them severely, the BBC's Subir Bhaumik says, quoting hospital officials.

Many of the injuries occurred in the stampede that followed the explosion, police sources said.

The cinema was screening a Bengali-language film at the time.

Assam's separatist rebel groups have targeted Bengali speakers in the past, accusing them of upsetting the ethnic mix in the area.

The rebels have also called for a boycott of Hindi-language cinema, which they regard as alien to their culture.