GUWAHATI (India) - INDIAN police interrogated about a dozen people on Friday over suspected links to serial bombings in the state of Assam that claimed 76 lives and left hundreds injured.

A total of 12 explosions - all within the space of an hour - shook the insurgency-hit northeastern state on Thursday, six of them ripping through crowded areas in the main city of Guwahati.

There was no claim of responsibility for the attacks, but speculation has focused on the rebel United Liberation Front of Asom (ULFA), which has been fighting for an independent homeland since 1979.

The ULFA issued a written statement formally denying any involvement.

Police said they were also investigating the possible role of Islamist fundamentalist groups active in Assam - such as the Bangladesh-based Harkat-ul-Jihad-al-Islami.

A senior police official said on Friday that detectives were making headway in their investigation.

'We should be able to zero in on the people or groups involved,' said the official, who did not want to be named.

Preliminary investigations showed some of the bombs had been strapped to bicycles and packed with incendiary material to trigger fires.

Assam Home Commissioner Subhas Das said 15 people had died of their injuries overnight, taking the death toll to 76, of whom 43 were killed in Guwahati.

Three other districts in western Assam were also targeted by the bombers.

The total number of injured stood at more than 300.

The powerful blasts, including one in front of the Guwahati District Magistrate's Court, reduced nearby vehicles to heaps of twisted metal.

UN Secretary General Ban Ki-moon, in New Delhi on an official visit, condemned what he called an 'act of terrorism targeting civilians', while Indian Prime Minister Manmohan Singh vowed to bring the bombers to justice.

'Such barbaric acts targeting innocent men, women and children only highlight the desperation and cowardice of those responsible,' Dr Singh said.

The attacks came six weeks after New Delhi was hit by a series of bombs in crowded markets that left more than 20 dead. Those blasts were claimed by a group calling itself the Indian Mujahedeen.

The emergence in recent years of an indigenous Islamist militancy has posed a fresh challenge to the Indian government, which has routinely blamed neighbouring rival Pakistan for organising attacks on its soil.

In the immediate aftermath of Thursday's bombings, the authorities slapped a curfew on Guwahati as local residents - blaming lax security for the attacks - took their anger out on police vehicles and threw stones at fire engines that were trying to battle a series of fires.

By Friday morning, the curfew had been lifted and there were fresh clashes as around 200 protestors confronted police in the city centre.

Other residents gathered around the blast sites to survey the damage.

'We are not going to be cowed by terrorists,' said pensioner Jiban Kakoti.

'We need to go about our normal work or else the terrorists will get the upper hand,' he added.

In the past two decades, more than 10,000 people have lost their lives to insurgency-linked violence in Assam, which is known for its tea, timber and oil reserves.

In January 2007, police blamed the ULFA for a wave of attacks in which 62 people were killed, many of them Hindi-speaking migrant workers.

Attacks have continued since peace talks between the ULFA and the government collapsed in 2006.

Insurgencies have wracked India's northeastern states - known as the 'seven sisters' - for decades. More than 50,000 people have been killed in insurgency-related violence in the region since India's independence from Britain in 1947. -- AFP