Chaos broke out at a religious festival in the Oromia region of Ethiopia, after police fired tear gas and warning shots to break up a protest.
At least 50 people were killed in the resulting stampede, according to reports from opposition party sources while the government has yet to give a precise death toll.
However, it did admit that "lives were lost" following the demonstration at which anti-government slogans were chanted and rebel flags waved.
"As a result of the chaos, lives were lost and several of the injured were taken to hospital," the government communications office said in a statement to Reuters.
"Those responsible will face justice."
Crowds shouted "we need freedom", and "we need justice" at the annual Irreecha festival in Bishoftu, about 40km south of Addis Ababa.
Community elders, deemed close to the government, were prevented from delivering their speeches at a religious festival, prompting police to fire tear gas that caused the stampede.
Merera Gudina, chairperson of the opposition Oromo Federalist Congress, told Reuters at least 50 people were killed when people fled after shots were fired.
Protesters at the festival were chanting slogans opposing the Oromo People's Democratic Organisation, one of four regional parties that make up the Ethiopian Peoples' Revolutionary Democratic Front, which has ruled the nation for quarter of a century.
The Oromia region has seen outbursts of unrest over the past two years, with anti-government feelings stoked by a series of land disputes.
According to New York-based Human Rights Watch, at least 500 people have been killed by security forces since the demonstrations began in November.
Though protests started among the Oromo, who are Ethiopia's biggest ethnic group , they later took hold in Amhara, the second largest ethnic group in the country.
Both groups say the ruling coalition is dominated by the Tigray ethnic group, which makes up only about 6% of the population.