Military spokesman Udaya Nanayakkara said an explosive device had been placed near an electricity transformer which went off around 6.15 am local time, but no one had been hurt.
Authorities had also recovered a one kilogram bomb from a telephone booth overnight, he said.
The bomb had also been found in the Nugegoda Kohuwela area at around 11.00 pm last night, a day that saw two explosions including a suicide bombing that killed 12 and wounded more than a 100 at the main train station in Colombo.
On Saturday 20 died from a bomb attack on a civilian passenger bus in the central part of the island.
The United States condemned the terror attacks, blaming it on the separatist Liberation Tigers of Tamil Tigers (LTTE).
"These reprehensible acts, which bear all the hallmarks of the LTTE, clearly targeted innocent civilians to foster an atmosphere of fear prior to Sri Lanka’s Independen