Rebels in Assam triggered a string of explosions Sunday, blowing up two oil pipelines and killing three people in separate attacks ahead of Independence Day celebrations.

Three people, including a woman, died when a powerful explosion rocked a crowded market near Boko village, 40 km from Assam's main city Guwahati, at 7.15 p.m.

"The explosion was very powerful and took place when the market was teeming with people. At least five of the 12 injured are in a critical condition," the official said.

Dipak Das, a witness, said: "The intensity of the blast was so severe that it tossed people up in the air and there were limbs strewn all over the place."

The injured were shifted to a hospital in Guwahati.

A major tragedy was averted Sunday when police found a powerful bomb planted on a railway track near Lanka in eastern Assam minutes before the Brahmaputra Mail bound for New Delhi was to cross the area.

"It would have been a tragedy if the bomb had gone off," a police official said.

In two separate attacks, rebels blew up two crude oil pipelines of state-owned Oil India Limited. A major fire broke out after a powerful explosion blew up a pipeline near the OIL headquarters at Duliajan at 7.30 p.m.

The 1,150-km pipeline supplies crude to a refinery at Barauni in Bihar.

In another incident, around 15 militants arrived at an area in eastern Assam's Sivasagar district where an underground oil pipeline is exposed at 12.30 a.m. and overpowered four security guards.

"The militants tied the guards to a lamppost and planted explosives beneath the pipeline before detonating them," the official said.

"There was a massive fire and at least half-a-dozen huts on the periphery of the area were burnt. The fire was brought under control after four hours."

OIL general manager Mohan Bhandari said supplies of crude were suspended after the blast in Sivasagar. "The blast was of a very massive intensity and the damage is extensive," he said.

Repairs had begun but it would take a day or two to resume supplies of crude, he said.

Militants also hurled a bomb at an Indian Army base in the western district of Nalbari early Sunday. The bomb missed the target and exploded near a telephone exchange.

Police suspect the blasts to be the handiwork of the outlawed United Liberation Front of Asom (ULFA) even though the rebel group has yet to claim responsibility for the attacks.

"The attacks are part of a campaign by ULFA to create terror and panic across Assam ahead of Independence Day. We had reports that ULFA would target vital installations and security forces beginning Saturday and they have done it," Assam Inspector General of Police Khagen Sharma told.

In a separate incident, an ULFA militant was killed when a group of three rebels clashed with a security patrol on the outskirts of Guwahati early Sunday.

In yet another incident, a paramilitary patrol had a miraculous escape when militants lobbed a grenade in western Chirang district Sunday night.

On Saturday night, three people were injured in two explosions in Guwahati.

ULFA, which has been fighting for an independent Assamese homeland since 1979, denied responsibility for the blasts Saturday.

Police officials said the stepped up violence was part of an annual campaign by the rebels ahead of Independence Day.

The run-up to the event has always been violent, with rebels striking vital installations including oil pipelines, trains, railway tracks, road and rail bridges and security forces.