The federal high court trial began in December 2005 following post-election violence that erupted during protests over polls six months earlier.
The opposition won an unprecedented number of parliamentary seats, but Meles Zenawi, the prime minister, held on to power. The opposition claimed the vote was rigged, and EU observers said the polls were marred by irregularities.
The trial has been widely condemned by international human rights groups as an attempt to silence Ethiopian government critics. The opposition leaders have claimed the trial is politically motivated.
The US has also condemned the convictions.
Sean McCormack, US state deparment spokesman, said: "Well, we're quite surprised, first of all, by the action that was taken by the government and very, very concerned."