Vehicles are set on fire by an angry crowd at a blast site in Guwahati, the main city of India's troubled northeastern" title="ad.&a.向东北(的)">northeastern Assam state Oct. 30, 2008.

Damaged cars are seen at a bomb blast site in Guwahati, the main city of India's troubled northeastern" title="ad.&a.向东北(的)">northeastern Assam state, Oct. 30, 2008.

NEW DELHI, Oct. 30 (Xinhua) -- At least 61 people were killed and over 300 injured in serial blasts in India's northeast state Assam Thursday, officials said.

As many as 12 bombs went off in less than an hour in Assam's major city Guwahati and the western districts of Barpeta, Kokrajhar and Bongaigaon. More than 300 people were injured, many of them are in a critical condition, a police spokesman said.

Five blasts targeted Guwahati's busy market areas. "The area was teeming with people, officer goers, shoppers and vendors when a very big explosion took place," a witness said, according to the Indo-Asian News Service (IANS).

Thick black smoke billowed into the sky as rescue workers rushed in to control the flames, the NDTV report showed.

It was one of the worst terror attacks in northeast India.

"Preliminary investigations show that the bombs were strapped on bicycles and the explosives were packed with highly inflammable substances that led to big fires soon after the blasts," said the Assam government spokesperson.

No militant group has claimed responsibility for the explosions.

Some officials of the Assam government pointed the suspicion towards the United Liberation Front of Asom (ULFA), Assam's militant group, but the group denied the charge, the report said.

"People should fight terrorism unitedly," India's Prime Minister Manmohan Singh said after the news came in.

India's political leaders, including President Pratibha Patil also condemned the terrorist attacks.

More than 600 people have died in terror attacks in the last six years in India, and 400 in the last two years alone, the IANS report said.

At least 17 people were killed and over 30 injured in a powerful bomb blast in another Indian northern city Imphal on Oct.21 evening.