Umarov, leader of the opposition group Sunshine Uzbekistan, was arrested 10 days ago in the Uzbek capital, Tashkent, and charged with economic crimes.
Sunshine Uzbekistan says he was arrested for political reasons.
Human Rights Watch says that when last seen by his lawyer -- three days after his arrest -- Umarov was naked in his cell, and in obvious distress. The human rights group says Umarov needs urgent medical attention and is at high risk of torture in his current detention facility.
The Uzbek government has not commented on the conditions of Umarov's detention.
The Sunshine Uzbekistan coalition has been calling for the dismissal of the government since a severe military crackdown on demonstrators in the eastern city of Andijon earlier this year.