Human rights watchdog Amnesty International has urged the Ethiopian government to free the 111 people on trial for treason and genocide.
The opposition leaders say the government controls the court
The trial of the opposition leaders, journalists and human rights activists has been adjourned until next Monday.
Amnesty said the genocide charges were "absurd". It relates to alleged attacks on ethnic Tigrayans during last year's protests over disputed elections.
Amnesty said the trial was a test of the independence of Ethiopia's judges.
Last week, UN High Commissioner for Human Rights Louise Arbour also called for the release on bail of the detainees.
'Prisoners of conscience'
Chief prosecutor Shimeles Kemal said he would produce evidence to show that the "defendants had the same criminal objective: to overthrow and dismantle the duly established government through violence."
Some of the detainees made allegations of abuse and judge Adil Ahmed ordered the prison authorities to investigate.
One defendant said he needed medical treatment after being beaten by the police.
More than 80 people were killed in protests over alleged fraud
Journalist Sisay Agena said the police had destroyed his office when they raided it, stealing 78,000 birr ($9,000).
All but three accused have refused to participate in the trial, saying they do not believe it will be fair.
Amnesty described them as "prisoners of conscience who have not used or advocated violence".
"This very worrying trial has major implications for human rights, media freedom and democratisation in Ethiopia," said Director of Amnesty International's Africa Programme, Kolawole Olaniyan.
"It will be a crucial test of the independence and impartiality of the Ethiopian judiciary."
During two waves of protests last year, more than 80 people were killed after security forces opened fire.
Several thousand people were arrested, although most have since been released.
Last week, a group of Western diplomats based in Addis Ababa called on the government to release all elected leaders, so they could help with the post-election reconciliation process.
Prime Minister Meles Zenawi, a Tigrayan, has denied the charges of election fraud and blames the opposition for starting the violent protests.
His government also points out that his government introduced multi-party elections to Ethiopia after years of military rule.
In the elections, the opposition made huge gains but say they were cheated out of victory.