At least 64 people were killed and 180 were injured in an airstrike on a market in Ethiopia's northern Tigray region, Agence France Presse (AFP) reported.
The Ethiopian military admitted carrying out the attack but said it targeted rebel fighters, not civilians.
Details of the bloody attack on Togoga town, 30 kilometres (18 miles) northwest of the regional capital Mekele, on Tuesday were slow to emerge partly because soldiers initially blocked emergency workers from accessing the area, the report stated.
By Thursday, hospitals in Mekele were treating at least 73 people, including young children.
A health officer told AFP at least 64 people had died.
"The air strike was in the market area, so many, many people were injured," said Mulu Atsbaha, an advisor to the Tigray regional administration on maternal and child health.
Survivors spoke of huge devastation as aerial explosions tore through the busy market around 1 pm, killing and injuring dozens, reducing nearby homes to rubble and burying people under the ruins.
But Ethiopia's military spokesman said rebel fighters dressed in civilian clothes, gathered in Togoga to celebrate "Martyr Day", were attacked.
"We do not accept that this operation targeted civilians," Colonel Getnet Adane told AFP.
"It is a clear fact that both the remnants of the TPLF and its militia dress in civilian clothes," he said, referring to the renegade former regional leadership.
The attack came as vote counting was under way following Monday's national elections in Ethiopia.
No vote was held in Tigray because of the conflict, and the region has seen an upsurge in fighting and rebel advances in recent days.
Prime Minister Abiy Ahmed sent troops to Tigray in November to oust the dissident regional leadership, promising a swift victory.
But nearly eight months later, fighting continues, which has triggered a humanitarian crisis with the UN warning 350,000 people are on the brink of famine, the report added.