Uzbek human rights activist Saidjahan Zainabiddinov, who witnessed last May's bloodshed in the eastern city of Andijan, has been put on trial.
Mr Zainabiddinov's family are worried about his whereabouts
His family said they have not been allowed to see him for five months.
Mr Zainabiddinov gave eyewitness accounts of what happened in Andijan to diplomats and foreign media.
The Uzbek government says nearly 200 died as it put down an armed Islamic uprising; others say 500 or more civilians may have been killed.
Human rights activists say Mr Zainabiddinov has been charged with undermining the constitutional system and assisting terrorism.
The Uzbek prosecutor's office has refused to give details of the charges and where and when the trial is being held.
A spokeswoman confirmed that the case had been handed over to the courts in December.
Human rights organisations and eyewitness reports say several hundred people were killed when the Uzbek security forces opened fire on unarmed demonstrators.
Mr Zainabiddinov stayed close to Andijan's main square during the crackdown, helping journalists and telephoning foreign embassies.
He saw demonstrators shot as troops surrounded them and sprayed the crowds with machine gun fire.
His version of events went directly against the government line, that those who died were armed Islamic militants trying to overthrow the state.
A series of trials have taken place over the events in Andijan, with at least 115 people reported to have been jailed so far.
In one case that drew international criticism, a court found 15 men guilty in November of trying to overthrow the government and set up an Islamic state.
They were sentenced to between 14 and 20 years in jail after pleading guilty.
Critics denounced the proceedings as a show trial.
Last week, the International Helsinki Federation for Human Rights said the Uzbek government's persecution of activists has grown worse since the bloodshed in May.