(Updates with more detentions, PM's quotes)

By Biswajyoti Das

GUWAHATI, India, Nov 1 (Reuters) - Indian police detained six people on Saturday after a little-known Islamist group claimed responsibility for bombings that killed 77 people in the troubled state of Assam.

Police said two cars and mobile phones used to detonate bombs in the remote northeastern state, including its main city Guwahati, had been traced to four men while the remaining two were picked up for possible links with the attackers.

A police officer said two of three cars used as bombs in Guwahati had been identified.

"One of the vehicle owners has been picked up for questioning," said Bhaskar Jyoti Mahanta, an inspector general of police.

Thursday's coordinated bomb attacks were the worst in India's turbulent northeast, home to more than 200 tribes and a focus of dozens of insurgencies connected with demands for autonomy or statehood.

The Islamic Security Force-Indian Mujahideen sent a mobile text message to a local television station claiming responsibility for the bombings.

"We, ISF-IM, take responsibility of Thursday's blast. We warn all of Assam and India of situation like this in future," the text message said.

Police say the group may have been seeking to avenge attacks on Muslim settlers by indigenous tribes that killed at least 47 people last month.

Ethnic tensions have simmered for decades in Assam where over the years Muslim settlers, mostly from Bangladesh, have moved to this Hindu and tribal-dominated region.

Security officials said they were investigating if the ISF-IM group was the same as an Islamic militant organisation formed in 2000 in Assam to avenge attacks by indigenous people.

But police say the text message could be a ploy to divert attention from the real attackers.

"You never know, it could be their diversionary tactic. We have not closed our options," Mahanta said.

WITNESS?

Police also were talking to a lawyer who said he had seen two men park a car near one of the blast sites in Guwahati and leave nervously. Police were preparing sketches of the pair.

Police were investigating the links of ISF-IM with the Indian Mujahideen, which first emerged in November 2007 and has claimed several major attacks across India this year.

The separatist United Liberation Front of Asom (ULFA) also is suspected, but security experts say the scale and sophistication of the attacks bore the hallmark of Islamist militant groups, and that ULFA may have played only a supporting or logistical role. ULFA has denied any involvement.

Police were trying to find out if ISF-IM was a front for Bangladesh-based Harkat-ul-Jihad al Islami (HuJI) that has often been suspected of attacks on Indian cities. HuJI has ULFA links.

Meanwhile, Prime Minister Manmohan Singh met families of the victims and survivors in Guwahati hospitals amid opposition charges his government had failed to stop a wave of bombings across India.

"Our government is not soft on terror," Singh told reporters outside Guwahati's main hospital.

A tight security blanket was draped over the state after a Hindu-nationalist group linked with India's main opposition Bharatiya Janata Party (BJP) called a day-long shutdown across Assam on Saturday to protest the bombings.

Police arrested more than 100 BJP supporters. (Writing by Krittivas Mukherjee; editing by Michael Roddy)

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