The Bahrain Centre for Human Rights said security forces were acting with a "culture of impunity" after Abdulla Yousif Mohammed and Zuhair Yehya Rashid were arrested for "attacking an official" despite being targets of violence.
The pair were arrested during a sit-in on August 14 after tear gas was used on protesters.
BCHR said they have been tortured and sexually harassed during their detention. Mr Rashid was taken to a military hospital because of his treatment.
The group called for the immediate release of the men, the release of all other political prisoners in Bahrain and for those responsible for excessive force to be brought to account.
Protests have continued since the country was gripped by the Arab spring in February 2011, with the Shi'ite majority demanding the Sunni al-Khalifa dynasty give up power.
On Friday, thousands marched outside capital Manama calling for democratic reforms, but police did not get involved.