In the initial aftermath, the head of the local police force, Raul Becerra, said the blast was caused by fireworks intended for the celebrations.
"This is an irresponsible and accidental consequence of people drinking alcohol," he told RPP, the Peruvian radio station.
Police sources later told Reuters they had not ruled out the possibility of a bomb attack.
Shining Path, the Maoist rebel group that plunged Peru into violence during the 1980s and 1990s, marked the 27th anniversary of its foundation this week.
Although it has been largely defeated, the group still carries out occasional guerrilla attacks.