They had refused to be defended in court and admitted responsibility for the unrest.
“The accused didn’t even express regret for their actions, which in itself is a crime under penal law,” Tetemke said.
Ethiopia has been criticised by international rights organisations over their detention, but Meles Zenawi, the prime minister, has called for no leniency.
A lawyer for the opposition members hoped that the group’s admission in planning a coup would result in a lighter sentence.
Earlier this year, the Ethiopian parliament approved a report which said 193 civilians and six policemen died during post-election violence in 2005.
Opposition groups called the report “baseless”, saying that it failed to hold the state accountable for the unrest.