New Delhi - The death toll in serial bomb explosions that struck India's north-eastern state of Assam rose to 74 Friday with several of the injured succumbing to their injuries overnight, police said.

More than 400 people were injured Thursday in 12 blasts that took place in quick succession - six in Assam's main city of Guwahati and six in the three western districts of Barpeta, Kokrajhar, and Bongaigaon.

The bombs were placed in cars, autorickshaws and drains in busy market areas. They were synchronized to explode within minutes of each other.

"The death toll has reached 74 - 43 in Guwahati, 20 in Kokrajhar and 11 in Barpeta," an official at the police control room in Guwahati said. Several of the injured were still in critical condition, he added.

Media reports said several people had been arrested in connection with the blasts, but the police would not confirm the reports.

"We are making good headway in our investigations and should be able to zero in on the people or groups involved in the serial bombings," an unnamed senior Assam police official was quoted as saying by IANS news agency.

Police said they suspected the separatist United Liberation Front of Asom (ULFA) of having a hand in the blasts with help from the Bangladesh-based Islamic militant outfit Harkat-ul-Jehadi Islami (HuJI).

However, the ULFA, in an e-mail sent to local media houses late Thursday, denied involvement.

The Assam police had gunned down seven suspected members of the HuJI in Assam's Dhurbi district in September. The police had then said that intelligence reports indicated that other similar cells had infiltrated into Assam with the intention of engineering blasts and Guwahati was one of the main targets.

"We had inputs about such blasts in Guwahati, but we did not anticipate such a serious attack," Assam Chief Minister Tarun Gogoi was quoted as saying.

Federal Home Secretary Madhukar Gupta, at a press briefing in New Delhi Thursday, said the blasts could be the handiwork of some north-eastern rebel groups having "external linkages" in an apparent reference to the HuJI's links with ULFA.

Federal Home Minister Shivraj Patil was scheduled to visit Assam Friday. Prime Minister Manmohan Singh, who is an elected member of parliament from Assam, was likely to fly to Guwahati Saturday.

A red alert has been sounded across the state. There have been many militant attacks in Assam in the past, but Thursday's attacks were the first time serial bombings have taken place.

There have been a number of major serial bombing in India in the past few months, many of them blamed on Islamic militant groups.

Two north-eastern state capitals - Agartala in Tripura and Imphal in Manipur - saw serial explosions earlier in October, in which 20 people were killed and more than 100 injured.

Serial blasts have also taken place in Jaipur, capital of north-western Rajasthan state; Ahmedabad, the main city of western Gujarat, and New Delhi since January.

UN Secretary General Ban Ki-moon, who is on a visit to India, condemned the blasts, saying there could be no justification for such indiscriminate violence.

"Ban strongly condemns this act of terrorism targeting civilians," his spokeswoman Michele Montas said in a statement. (dpa)