United Nations High Commissioner for Human Rights Louise Arbour has strongly criticised Ethiopia's human rights situation, calling it "worrying".
The opposition leaders say the government controls the court
Mrs Arbour, who is in Ethiopia visiting prisons, said conditions she had seen were "rudimentary" and "harsh".
She said it was not right that detainees had been held in custody for a year without bail.
Opposition leaders and journalists are among 129 people who have been denied bail after being accused of genocide.
Mrs Arbour also said she was surprised that such serious charges, which also include treason, were brought after a series of opposition protests following allegations of election fraud a year ago.
Thousands were arrested following violent protests
On Tuesday, 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.
Their trial is expected to resume next week.
Several thousand people were arrested after the protests, in which more than 80 people were killed, after security forces opened fire.
"It is worrying that at best we are in [a] state of stagnation, especially regarding political and civil rights which are in decline after months and years of hope," she told the AFP news agency.
During her visit to Kaliti prison, Mrs Arbour met some of the detained Coalition for Unity and Democracy (CUD) leaders.
"I was particularly struck and distressed to see in the women's quarter of the prison... children from infants to five-year-olds, whose mothers are incarcerated, running around in prison," she said.
Mrs Arbour also held talks with Prime Minister Meles Zenawi but said she had received no promises of leniency if the opposition figures, civil rights activists and media workers are convicted of crimes which can be punished by death.
Mr Meles has denied the charges of election fraud and blames the opposition for starting the violent protests, in which members of his Tigray community were allegedly targeted.
His government also points out that his government introduce 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.