The Monday night attack was carried out by a motorised paraglider and targeted a village in the central Sagaing region.
The village was in the midst of celebrations of a Buddhist festival and a rally calling for the release of political prisoners held by Myanmar's military government, the reports said.
Myanmar is in a civil war that began after the army seized power in February 2021 from the elected government of Aung San Suu Kyi.
More than 100 people from Bon To and nearby villages had gathered at the village's primary school compound Monday evening for an oil lamp lighting ceremony to mark the end of Buddhist Lent and to call for the release of political prisoners, including Suu Kyi, said a member of a local resistance group who attended the event.
A motorised paraglider dropped two bombs at about 7:15 pm (local time), killing an estimated 20 to 40 people, including children, villagers and members of local political activist groups and armed anti-military groups, said the resistance fighter, who spoke to The Associated Press.
More than 50 others were wounded, including himself, he added.
"The sickening reports emerging from the ground in central Myanmar following a night time attack late on Monday should serve as a gruesome wake-up call that civilians in Myanmar need urgent protection," the human rights group Amnesty International said in a statement.
The resistance fighter said an alert had been issued through a network of mobile phones and walkie-talkies that had tracked the paraglider from the army's northwestern military command in Monywa, about 25km north of Bon To.
A resident who also attended Monday's ceremony said the crowd began to disperse after hearing reports of an approaching paraglider, but it arrived sooner than expected and dropped bombs while people were still in the school.
The resident, who helped in rescue efforts after the attack, said at least 24 people were known to have been killed, though the death toll could be higher as the victims' family members and rescue workers worked independently to collect the bodies.
Both witnesses said the paraglider returned to the scene at around 11pm, and dropped two more bombs without causing additional casualties.
The military has not acknowledged attacking the area.
More than 7300 people are estimated to have been killed by security forces since the army's 2021 seizure of power, according to figures compiled by non-governmental organisations.
Myanmar's military also uses Chinese and Russian-made combat aircraft and helicopters, but since late last year has stepped up the use of low-tech motorised paragliders in what is believed to be partially an effort to save money.
Resistance forces lack effective defences against any kind of air attacks.
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