Jawar Mohammed, a former media mogul and member of the opposition Oromo Federalist Congress, is one of 24 suspects recently charged with terrorism and other offences in connection with violence that left up to 239 people dead in June and July.
The attorney general's office unveiled the charges in a Facebook post over the weekend without informing the suspects or their lawyers.
On Monday Jawar and 17 other suspects appeared in court, where they were presented with documents detailing the charges, according to Jawar's lawyer, Tuli Bayyisa.
Though they did not enter formal pleas, Jawar delivered a scathing statement challenging the court's integrity.
"The reason why I've been detained is only because we've challenged the government. The government understood that they cannot compete with the opposition parties," Jawar said, according to Tuli.
"I'm really very proud to be charged under the antiterrorist law because the government knows, the public knows, and me myself I know 100 percent that I'm not a terrorist. I have never committed any kind of crime."
Feelings of marginalisation
The charges -- which could bring life imprisonment -- relate to violence that erupted after the shooting death in June of Hachalu Hundessa, a popular singer from the Oromo ethnic group who gave voice to Oromo feelings of political and economic marginalisation.
Jawar is among more than 9,000 people caught up in subsequent mass arrests that have stoked criticism that Prime Minister Abiy Ahmed is seizing on the unrest to silence political opponents and critics.
Abiy, winner of last year's Nobel Peace Prize, is Ethiopia's first Oromo leader, but he faces intense criticism from Oromo nationalists like Jawar who accuse him of being a poor advocate for their interests and behaving like a dictator.
Putting Jawar on trial risks fuelling violence in Addis Ababa and the surrounding Oromia region.
"Unless prosecutors provide convincing evidence that the Oromo opposition leaders are guilty of these grave charges, then, if they are convicted, there will be a widespread public perception, especially in Oromia, that the Ethiopian government is once again engaging in politicised prosecutions," said William Davison, senior analyst with the International Crisis Group.
This could undermine national elections expected next year that are seen as a potential milestone in Ethiopia's democratic transition, Davison added.