Two Sunni clerics from the Association of Muslim Scholars spoke out against conditions in military detention centres run by the US-led forces.
"We have received messages from inmates at Um Qasr (Buka detention centre on the border with Kuwait) describing their suffering during this hot weather," said Ahmad Abd al-Gafur Samarrai, addressing a crowd at the Um al-Qura mosque in Baghdad.
Samarrai called on the United Nations to intervene on behalf of the detainees in Buka and demanded that they be freed, reported an Aljazeera correspondent in Baghdad.
"The United Nations must do something because it granted legitimacy to the occupation, but this legitimacy has been lost due to the actions that have taken place," said Samarrai in reference to the thousands of Iraqis locked away on suspicion of involvement in the persistent fighting that dogged the 14-month US-led occupation.
In another Sunni mosque in the capital, Shaikh Hassan Samarrai criticised US troops along similar lines.
Both clerics also called for a non-violent protest sit-in organised by the Iraqi Islamic Party at the doors of the detention centre in Um Qasr.
Meanwhile in Ramadi, another Sunni cleric called on his followers to take arms against US forces in Iraq and threatened to turn the city into a "graveyard" for American troops.
"We have received messages from inmates at Um Qasr describing their suffering during this hot weather"
Ahmad Abd al-Gafur Samarrai,
"I ask US President Bush to withdraw from Iraq or else Ramadi will become a graveyard for US soldiers," declared Shaikh Akram Ubayed Furaih at weekly prayers in the city, 100 km west of Baghdad.
"I call upon my brothers, the Shia, and on all other religious groups to embark on a jihad against the US military to force them out of Iraq," said the cleric, who spent three months in a prison after being arrested by the US military, and whose home was also raided last week.
"I urge all the Iraqi people to fight against the Americans," said the cleric, among the most respected figures in Al-Anbar province which encompasses Falluja and Ramadi.