A special military court in Bahrain during the weekend had trials for opposition members of Parliament and legal professionals without notifying legal teams or their family members, Human Rights Watch said.
Bahrain is rounding up scores of opposition and healthcare workers as part of a crackdown on a Shiite uprising against the Sunni monarchy.
"Most defendants hauled before Bahrain's special military court are facing blatantly political charges and trials are unfair," said Joe Stork, deputy Middle East director at Human Rights Watch, in a statement.
Bahrain is host to the U.S. Navy's 5th Fleet and is seen as a key U.S. ally in the region. Crown Prince Salman bin Hamad al-Khalifa met last week with U.S. officials, including President Barack Obama and Secretary of State Hillary Clinton, to garner support for an apparent dialogue with opposition forces.
"The crown prince may be sincere in his efforts to promote dialogue but what good is that while back home the government is crushing peaceful dissent and locking up people who should be part of the dialogue," said Stork.