Ibrahim Sharif, a Sunni activist who spent four years in jail over his involvement in 2011 anti-government protests, has been on trial since August for "promoting political change through forceful means".
His re-arrest in July followed a speech he made at a ceremony for a victim of the unrest that has shaken Sunni-ruled Bahrain, which has a Shiite majority.
The head of the Al-Wefaq Shiite opposition bloc, Ali Salman, was sentenced in June to four years in prison after he was convicted of inciting disobedience and hatred.
An appeals court is reviewing that conviction while the prosecution is demanding the annulment of his acquittal in the charge of plotting to overthrow the regime and a tougher sentence.
The United States and Britain "are fully aware of the gross unfairness of Salman's trial and the content of Sharif's peaceful speeches, and this should give them good reason to call publicly for an end to their prosecutions and their immediate release," HRW deputy Middle East director Joe Stork said.
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Both regime opponents will face new hearings in their trials next week.
"Salman and Sharif have consistently supported peaceful political reform and should be at a negotiating table with Bahrain's government, not languishing behind bars," said Stork.
"Countries that say they support Bahrain's reform process should make this point publicly," he said.
Tiny but strategic Bahrain, which is home to the US Navy's Fifth Fleet, has been rocked by unrest since a 2011 Shiite-led uprising demanding a constitutional monarchy and a more representative government.
Despite a sweeping crackdown, protesters continue to clash frequently with security forces in Shiite villages outside Manama.
At least 89 people have been killed in such clashes since 2011, while hundreds have been arrested and put on trial, human rights groups say.