An Uzbek woman says she saw government troops open fire on unarmed civilians during protests in Andijan in May.
The 15 defendants are accused of inciting the bloody protests
Her testimony, at the trial of 15 men alleged to have led the revolt, contradicts government accounts of what happened during the unrest.
The Uzbek government says nearly 200 people, mostly "terrorist organisers", died when security forces put down an armed Islamic uprising.
Human rights groups say 500 or more civilians may have been killed.
Makhbuba Zakirova told the court that she saw soldiers shooting at people waving a white flag.
"Even Hitler did not do such things," she said.
Mrs Zakirova said that after speaking out in court, she feared for her life and freedom.
Correspondents say her statement undermines three weeks of testimony in what many foreign observers had dismissed as a show trial.
All 15 accused men have pleaded guilty to charges that they were trying to overthrow the Uzbek government and set up an Islamic state, and they have all given long and detailed testimonies.
'Telling the truth'
Mrs Zakirova, 33, said she mingled with anti-government protesters in the city square while walking with her children.
She said she stayed out of curiosity when she heard Uzbek President Islam Karimov was supposed to talk with the protesters.
"There were people in helmets everywhere. I twice saw soldiers shooting from military vehicles. The shooting was intense," she said, the Reuters news agency reported.
Mrs Zakirova was interrupted by the prosecutor, who asked: "Do you realise what you are saying? Are you sure?"
She replied: "Are you going to arrest me now? I was telling only the truth, and you yourself asked me to give a truthful testimony... I am only saying what I saw."
Earlier this week, the Uzbek authorities denied reports that illegal methods may have been used to force confessions from the 15 men on trial.
A former Uzbek interior ministry employee told the BBC that beatings or psychotropic drugs were often used to force confessions.