Nasir Ayif was taken into custody in the northwestern Jamiah neighbourhood in response to the party's opposition to the US-led offensive on Falluja, party official Iyad al-Samarrai said.
"This action is a kind of punishment to the Islamic Party because we object to what is happening in Iraq, especially Falluja, and to the security policies adopted by the Americans and the Iraqi government," al-Samarrai was quoted as saying.
He told Aljazeera these kinds of arrests were always carried out "under the pretext of possession of weapons and explosives or carrying out anti-US actions".
Many of these accusations are false, he said, adding that "the arrests comes as a punishment for the people or the organisations they represent".
Last week, the Iraqi Islamic Party, one of the strongest Sunni political parties in the country, had withdrawn from the interim government to protest against the US assault on Falluja, saying it "has led and will lead to more killings and genocide without mercy from the Americans."
The detained official has been
assisting in Falluja relief aid
Al-Samarrai said the detained official "spent the past few weeks devoting all his efforts to offering humanitarian aid to the families fleeing Falluja".
"His arrest may be an expression of dissatisfaction with the Islamic party's anti-US policies," the party official said.
Al-Samarrai said interim Iraqi Prime Minister Iyad Allawi had previously assigned Ayif to oversee the Falluja negotiations.
"Therefore, I call on Iyad Allawi to immediately do something and solve this issue."