In a letter posted on an independent Uzbek website, the group urges international organizations to secure the release of Umarov, who is serving a prison sentence on charges it says are "entirely fabricated."
Umarov is the leader of the Sunshine Uzbekistan opposition coalition. He was arrested in Tashkent on October 23, 2005.
A court in March sentenced him to at least 10 1/2 years in jail on charges of tax evasion, money laundering, and embezzlement. His sentence was subsequently reduced by three years.
The Initiative Group of Independent Rights Defenders says that Umarov is serving his sentence in the central town of Kizil-Tepe, that his health is deteriorating, and that no one is allowed to visit him.
RFE/RL Central Asia Report
SUBSCRIBE For regular news and analysis on all five Central Asian countries by e-mail, subscribe to "RFE/RL Central Asia Report."