Bahraini security forces and prison guards used batons, teargas, pepper spray and stun grenades against inmates of the Dry Docks prison, injuring at least 40 convicts, according to the Bahrain Centre for Human Rights (BCHR).
The country's director of reformation and rehabilitation, Col Mohammed al-Hussaini, confirmed that a number of prisoners rioted and were hurt during security guards' attempts to restore order.
"They tried to break doors, the police interfered and restored order," he tweeted through the Ministry of Interior account.
However, the BCHR reported that families of the detainees received calls from the prison suggesting that political inmates had been tortured and abused. The clashes erupted when prison guards began a sudden search of the prison cells.
"They humiliated, tortured and abused political detainees by cursing, severely beating with batons, using pepper sprays and threatening them," BCHR said.
"Some of the prisoners defended themselves [which] resulted in the security forces using more violence. The BCHR received confirmed information from sources that the prison guards and security forces used stun grenades and teargas inside closed cells that resulted in at least 40 injuries."
The human rights organisation noted that the political section of the prison - where prisoners tried under the controversial terror law, including a pro-democracy activist, are being held - is notorious for guards mis-treating prisoners.
"The use of crowd control weapons inside the prisons is specifically alarming, as this violates many local and international laws," said Maryam al-Khawaja, acting president of the BCHR. "We need urgent actions from the United Nations and the ICRC to look into the situation of these political prisoners.
"We reiterate that the further deterioration of the human rights situation both inside and outside prisons is the direct result of the lack of local and international accountability for the Government of Bahrain. This is especially in regards to the international impunity granted to the authorities in Bahrain by their closest allies, namely the United Kingdom and the United States."
Protest rallies demanding democratic reforms were organised on 14 August to mark the anniversary of Britain's withdrawal from the tiny Gulf kingdom in 1971.
Police fired teargas and birdshot at protesters across the island.