Ethiopia's opposition leaders are among 129 people who have gone on trial for treason and attempted genocide.
The opposition leaders say the government controls the court
None of the accused answered questions, saying the trial was political. Some wore black t-shirts and put their hands over their mouths when asked to plead.
The charges relate to last November's street clashes that killed 46 people over disputed polls in May, won by Prime Minster Meles Zenawi's party.
The opposition claimed the polls had been rigged - charges Mr Meles denies.
One of the Coalition for Unity and Democracy (CUD) leaders, Bertukan Medeksa, a former judge, said the court was controlled by Mr Meles, who had already found them guilty.
Some diplomats told the BBC they feared that the trial could drag on for a long time.
They pointed out that the trial of those accused of mass murder under the former regime of Haile Mengistu Mariam is still underway 15 years after it began.
Thirty-five of the accused are abroad and are being tried in absentia.
As well as CUD officials, the accused include journalists and civil rights activists.
The genocide charges relate to alleged attacks on members of Mr Meles' Tigrayan community.
Mr Meles has come under strong international criticism amid allegations that his government is reneging on democratic commitments.
Some 8,000 people have been freed since opposition arrests after two waves of poll protests in June and November in which more than 80 people died.