They said guerrillas of the United Liberation Front of Asom (ULFA) were behind Sunday's attacks which came four days before Republic Day, which the rebels want to boycott with a general strike in the region.
The guerrillas threw grenades at security personnel in Guwahati, the capital of Assam state, killing one policeman, and triggered explosions at about nine oil and gas pipelines which caught fire and disrupted supplies to the state's four refineries, police said.
The ULFA is fighting for an independent homeland in Assam. It blames New Delhi for taking away the region's considerable oil resources without helping it to develop.
India celebrates Republic Day on 26 January, marking its founding as a republic in 1950 with parades in key cities to show off its military might and cultural diversity.
A senior official of Indian Oil Corp (IOC), which owns the refiners, said: "The refineries have enough stock to run for a week, but if they carry out more attacks on pipelines in coming days it will definitely affect our operations."
Rebels killed another policeman in a gun battle in central Assam on Sunday evening.
Khagen Sharma, Assam's inspector general of police, told Reuters: "The attacks are the handiwork of the ULFA. Reinforcements have rushed to several areas of the state to tighten security and catch the militants."
Assam is known for its tea
plantations and oil refineries
Last week, Paresh Barua, the ULFA's military commander, said the group had demanded about $113 million from the state-run Oil and Natural Gas Corp (ONGC).
He said that similar demands on other government oil companies in the state would soon follow.
India's seven northeastern states are home to more than 200 ethnic and tribal groups and are beset by at least two dozen other insurgencies.
In 2004, at least 22 people, many of them children, were killed and dozens wounded in bomb and grenade attacks during independence day ceremonies in the region, which authorities blamed on the ULFA.