Two minors, a woman and an elderly man were injured in the incident that took place at Bhaladmi Tol area in Itahari sub-metropolitan city, some 366 kms from Kathmandu, police said.
The Improvised Explosive Device (IED) went off near the podium of a meeting convened by minister of state Sumitra Devi Chaudhary.
The one-year-old child was said to be in a serious condition as he has sustained injuries on his head. The condition of the 16-year-old boy was stable but not out of danger. The woman and the elderly man sustained minor injuries, the police said.
The blast occurred one week before the second phase of the provincial and parliamentary elections on December 7.
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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 of the polls while the rest will exercise their franchise during the second phase on December 7.
Prime Minister Sher Bahadur Deuba visited the Tribhuvan University Teaching Hospital in Kathmandu and met with the victims of yesterday's bomb blast.
He asked the hospital to make all necessary arrangements for the treatment of injured people.
The elections are being seen as the final step in Nepal's transition to a federal democracy following a decade-long civil war till 2006 that claimed more than 16,000 lives.