New Delhi - At least 54 people were killed and more than 200 injured Thursday in 11 bomb blasts triggered by suspected separatist militants in India's north-eastern state of Assam, officials and news reports said.

Officials said there were 11 blasts within 20 minutes, the first at 11:20 am (0550 GMT).

Four blasts took place in the state capital, Guwahati; three in Barpeta district; three in Kokrajhar district; and one in Bongaigaon district, all in western Assam, the IANS news agency reported.

"According to the information we have, 54 people died in the blasts and 224 were injured," an official at the police control room in Guwahati said by telephone. "Twenty-three victims were in Guwahati alone."

The doctors said the death toll could rise as several of those injured in the high-intensity explosions were in critical condition.

Television footage showed thick black smoke billowing from destroyed shops and mangled vehicles as people screamed and ran in panic.

The Guwahati blasts took place in crowded markets like Fancy Bazar, Pan Bazar and Paltan Bazar as well as the busy Ganeshguri area, where heavy security is generally deployed.

"The area was teeming with people, office-goers, shoppers and vendors when a very big explosion took place," Arindam Das, a witness at the blast in Ganeshguri, told the IANS news agency.

"I saw at least six dead bodies while more than 30 people were lying on the ground and bleeding," Das said.

The bombings sparked protests and street violence with angry mobs attacking police teams in Guwahati, saying they had failed to provide adequate security.

"Crowds went on a rampage, pelting stones at police and trying to set police vehicles and even fire tenders ablaze," another witness, Saurabh Chowdhary, told news channels. "There is utter lawlessness."

The state administration placed police on red alert, later imposed a curfew in sensitive areas in Guwahati and deployed additional paramilitary patrols.

"A red alert has been sounded across the state," Assam Chief Minister Tarun Gogoi told reporters. "It is a designed and planned attack carried out to spread terror."

No militant or terrorist group had claimed responsibility for the attacks.

Several militant outfits with demands ranging from autonomy to secession are active in the state.

More than 20,000 people have been killed in separatist violence in Assam since 1979.

Local media outlets reported that police suspected the United Liberation Front of Asom (ULFA), a major separatist group in Assam, of having carried out the attacks with help from a Bangladesh-based Muslim militant outfit, the Harkat-ul-Jehadi Islami

Earlier this month, two people were killed in four bombings that local police blamed on Muslim militants based in neighbouring Bangladesh.

"It is very early to make a conclusion, but ULFA has a history of triggering serial blasts," Assam Health Minister Himanta Biswa Sarma told reporters.

Indian security agencies said they were investigating which militant organization was behind the bombings.

"We cannot say anything about who is behind the attacks until we get the complete investigation report," junior Home Minister Sri Prakash Jaiswal told reporters.

"Assam has been a disturbed state and has witnessed such incidents every one or two months over the last 30 years," he said. (dpa)