Abraha Tetemke, the chief prosecutor, said: "According to the country's penal code, maximum punishment should be dealt to parties found guilty of plotting against the constitution."
More than 100 opposition figures were put on trial on charges of plotting a coup following elections in 2005 which the ruling party won but the opposition claims were rigged.
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.
The figures compiled by the inquiry were three times higher than the government's official death toll of 54.
Opposition groups called the report "baseless", saying that it failed to hold the state accountable for the unrest.