At least five persons were killed on the spot and more than a dozen injured in a powerful blast at the busy Machkhowa bazaar in the heart of the Assam capital today, with the police holding the outlawed ULFA responsible for it.The blast took place at around 4:15 pm in the vegetable market just behind Pragjyoti, a cultural complex sponsored by the Indian Tea Association (ITA) where about 400 people, most of them on their way home from office, were shopping. Two of the five killed have been identified as vegetable vendors in front of whose stall the explosion had occurred. The other victims were yet to be identified when this report was filed. This is the first major incident since ULFA entered into an indirect talks process with New Delhi in October last year. It is also the first incident since the Congress-led coalition government took over last month.Eyewitnesses said the blast took place in front of a vegetable stall, with the entire area becoming dark with smoke and dust. ‘‘I was buying papayas when I heard an explosion only a few metres away. I also saw dust and smoke and some objects flying out,’’ said Nur Hussain, a resident of a nearby locality who had a narrow escape.While the police arrived within 10 minutes of the blast, all the injured and the dead were taken to the GMC Hospital, where five were declared brought dead. ‘‘We had prior information that ULFA would unleash a spree of violent incidents for three days beginning today, and security had been beefed up at all vital installations, including oil pipelines, bridges and government buildings,’’ Assam Police Director-General D N Dutt told The Indian Express, quoting interrogation statements of Mrinal Hazarika, a hardcore ULFA militant who was arrested in Siliguri on May 18. Dutt said Hazarika had told policemen during interrogation that ULFA C-in-C Paresh Barua had instructed all ‘‘battalion commanders’’ of the outfit to strike at vital installations and security personnel on June 9, 10 and 11. In fact, 10 persons were injured in three similar explosions that took place at Haibargaon, Dhubri and Mangaldoi late last night, all triggered by ULFA.‘‘ULFA has been resorting to violent acts every time a discussion between the Central Government and the People’s Consultative Group takes place,’’ Dutt said, reminding about the previous two rounds of discussions, which were preceded by such violence. ‘‘It could be some arm-twisting tactic that the outfit resorts to,’’ the Assam DGP added.