In Baghdad, Shaikh Mahdi Sumaydai, a Sunni, denounced the idea backed by the US and interim Prime Minister Iyad Allawi.
"We don't want (Arab and Muslim) forces to shield the occupier because we don't want to see Muslims killed," he said.
Muqtada al-Sadr, the Shia leader, also criticised the move at a sermon in the main mosque in Kufa, south of Baghdad.
"I advise all countries that want to help Iraq, not to send forces here... If such forces come to Iraq, they will be seen as collaborators of the occupation," he said.
In Fallujah, Shaikh Ihsan al-Duri, a Sunni, said he opposed "any Arab or Muslim force in Iraq".
"I call on the occupation forces to release honest resistance fighters, otherwise we will unleash civil disobedience"
"Such forces could have served before (the occupation) but never now," he told worshippers at the Rawi mosque in the stronghold of Iraqi resistance.
Al-Sadr, in his sermon, went on to criticise Allawi's interim government for failing to restore basic services and safeguard security.
"The government said it wants to give Sadr's movement a chance, but several of his leaders are in prison and others have been threatened with death for refusing to accept occupation," he said.
"I call on the occupation forces to release honest resistance fighters, otherwise we will unleash civil disobedience," he said.
Al-Sadr spearheaded a resistance movement against the US occupation earlier this year in central and southern Iraq.