Abdulhadi al-Khawaja appeared frail, but spoke in a clear voice as he and others claimed on Tuesday that they suffered abuses and torture in custody and demanded their release.
Al-Khawaja and seven other activists were sentenced to life in prison last year by a military-run court. It was part of crackdowns by Bahrain's Sunni rulers against a Shiite-led uprising calling for a greater political voice in the Gulf kingdom.
A civilian court retrial was ordered in April for a total of 21 people convicted of anti-state crimes.
The court adjourned the case until May 29.