The bomb exploded near Ukhrul police station at about 8.30 pm last night, but no one was injured.
The bomb might have been planted by militants, they said.
Ukhrul, a Naga dominated district, was once a stronghold of Naga militant outfit NSCN-IM, but it has been peaceful for the past many years since the outfit entered into peace talks with the central government in 1997.