The police said that 30 suspects have been arrested in connection with the incident which occurred at Chapali area in Budhanilakantha municipality, 10 kms north of Kathmandu.
The injured including Nepali Congress candidate Gagan Thapa were rushed to the Tribhuvan University Teaching hospital here.
Thapa, a former health minister who is contesting election from the Kathmandu constituency Number 4, has been discharged from the hospital.
Election campaign for Thursday's second phase of parliamentary and provincial elections will end at mid-night tomorrow.
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The blast occurred three days before the second phase of the provincial and parliamentary elections on December 7.
Around 65 per cent votes were cast on November 26 in the historic first round of elections that many hope will bring the much-needed political stability to the Himalayan nation.
Of the total 15.4 million voters, 3.19 million were eligible to cast their votes in the first phase while the rest will exercise their franchise during the second phase.