Reports from the Tbilisi courtroom say Vladimir Arutyunian is refusing to testify unless Human Rights Watch observers attend the hearings.
Arutyunian, a 27-year-old ethnic Armenian from Tbilisi, was arrested on 21 July following a shoot-out that left one Georgian policeman dead.
Georgian authorities have released police videos showing Arutyunian purportedly confessing to throwing the grenade and acknowledging that he intended to injure U.S. President George W. Bush.
The incident reportedly took place when Bush was addressing Tbilisi residents during a visit to the Georgian capital on 10 May.
Investigators say the Russian-made explosive malfunctioned and failed to detonate.
Arutyunian faces life imprisonment.
(Imedi TV/Novosti Gruziya/Reuters)