Though security forces were on full alert in view of the I-Day boycott call given by over 15 different militant groups in the North-East, ultras managed to strike in Tripura and Manipur, gunning down at least 36 people in the two states in a span of less than 24 hours since yesterday.Thirty people, mostly Bengali-speaking settlers, have been massacred in three different incidents since last evening in Tripura. It was the worst-ever incident of militant violence in the two-and-a-half-decade-old history of insurgency in the Northeastern state.Meanwhile, Independence Day was observed under heavy security in Manipur, Tripura, Meghalaya and Assam, with the chief ministers of the respective states once again making a fervent appeal to the militants to come forward for a peaceful negotiated settlement of their grievances.The worst violence was seen in Totabari village of Khowai subdivision of West Tripura district where militants belonging to the All Tripura Tiger Force (ATTF) attacked the village around 9.30 pm last night, killing seven on the spot. Seven others died on their way to hospital, official sources in Agartala said over the phone.Around the same time, another group of armed ATTF cadres struck at village Baralunga, killing 12 people, including women and children. Five others who sustained injuries died later. In both cases, the militants arrived in military fatigues, dragged out people irrespective of sex, and opened indiscriminate fire on them, police said in Agartala. In three other incidents, security forces including the BSF also came under attack though there were no casualties. The ATTF was one of the 15 insurgent groups of the North-East which had called for a boycott of the Independence Day celebrations, saying ‘‘India was forcibly occupying’’ the region.Five BSF jawans were injured at Khantlang, close to the Bangladesh border, shortly after dusk yesterday when a state police patrol party was fired upon at Kapatoli in South Tripura district.Police sources, however, suspected the hand of the National Liberation Front of Tripura, another underground group, in the two attacks.The ruling Left Front and Opposition Congress have called a dawn-to-dusk Tripura bandh tomorrow to protest the killings. LF spokesman Gautam Das said the bandh was called to press for their five-point charter of demands, including deployment of Army in disturbed areas and increase in the BSF strength along the 856-km-long Indo-Bangla border.In Manipur too, Independence Day celebrations were marred by violence as different militant groups started attacking civilians and security personnel in different districts of Imphal valley starting Thursday morning, leaving at least eight people injured.While six people were killed on the spot when militants triggered a powerful explosion as a bus was crossing a bridge on the Lilong in Imphal (West) district yesterday, two others were killed in incidents elsewhere. At least 15 passengers of the ill-fated bus are battling for life at RIMS Hospital, sources said. Meanwhile, bus services have been suspended in the Indo-Burma-Sugnu road after three transport organisations announced a seven-day ceasework in protest against the blast at Lilong.