The bus was attacked at Satungal in the outskirts of the capital while it was heading from here towards Gaur in southern Nepal.
According to police, those injured in the incident have been admitted to a local hospital for treatment.
At least 37 people were injured in different parts of the country in the attacks by people trying to foil the November 19 Constituent Assembly polls.
Some 19 vehicles were attacked with petrol bombs over the past few days.
The Maoist party had claimed that their poll boycott was peaceful.