Chaos erupted after police charged stone-throwing protesters and fired tear gas, triggering a stampede,
The regional government said in a statement that 52 people had died after panic swept through the crowd of tens of thousands of people, with some festival-goers falling into a deep ravine.
Merera Gudina, chairman of the opposition Oromo Federalist Congress, told AFP he believed there had been many more fatalities than the number officially announced.
"Bodies are being collected by the government. But what I hear from people on the ground is that the number of dead is more than 100," said Gudina.
On Monday the national flag was being flown at half-mast in government institutions and regular radio programmes were replaced with music as the Horn of Africa nation mourned those killed.
The regional government blamed "irresponsible forces" for the disaster.