High alert has been sounded in Guwahati and upper Assam following two consecutive bomb blasts.Police said,militants exploded a grenade at Digboi town. Senior Superintendent of Police A.P. Tiwari said,security has been intensified following intelligence inputs that cadres of ULFA opposed to peace talks between the rival faction and New Delhi is trying to enter the city with two explosive laden Maruti vans.People have been asked to alert police about suspicious Maruti vans.Tiwari said,checking of vehicles and frisking of people across the city have been intensified and all entry and exit routes are under strict vigil.