"The power which is hostile to Islam...is plotting to divide this nation along ethnic, denominational and territorial lines," said al-Qaradawi.
He did not specify the power in question.
Ayatollah Mohammad Ali Taskhiri, who heads an Iranian body that seeks to unify followers of Islam's various branches, accused the United States and Israel of fuelling sectarian divisions in Iraq and Lebanon.
"The enemy's exploitation of the natural differences of opinion between Shia and Sunnis...represents a response to September 11," Taskhiri said, referring to the attacks in the United States by members of al-Qaeda.
Iranian officials often accuse the United States of stoking sectarian tension in Iraq where most people are Shia, like most Iranians. Washington accuses Iran of contributing to the violence in Iraq.
The three-day forum in Doha has drawn more than 200 delegates from over 40 countries including Ekmeleddin Ihsanoglu, head of the Organisation of the Islamic Conference, Ali Gomaa, the Grand Mufti of Egypt, and Mahmoud Hamdi Zakzouk, Egypt's religious endowments minister.