Police say the suspected rebels detonated the bomb at a Government office in Kathmandu.
A policeman has been wounded when two rebels shot at him before they entered the building in central Kathmandu and planted the bomb.
Splinters from the blast wounded a passing motorcyclist.
Earlier, the guerrillas fighting to topple Nepal's constitutional monarchy set off a bomb at an empty police post on the outskirts of the city.
The blasts are seen as an attempt to scare people and ensure the highway blockade is not violated.