Iraqi Shiite cleric Muqtada al-Sadr called for a massive prayer service and a peaceful demonstration in Baghdad next week against the U.S.-led presence in Iraq.
Worshippers demonstrate a proposed security deal between the United States and Iraq.
In a letter, al-Sadr called for a pan-Muslim Friday prayer sermon in central Baghdad's Firdous Square next week, instead of separate services in individual mosques, reported CNN.
After the prayers, al-Sadr said participants should protest peacefully against the proposed security agreement under negotiation between the Iraqi and U.S. governments.
"Let them all be united to foil the signing of the agreement that wants to sell Iraq to the occupier just like our holy lands in Palestine and other Arab and Islamic lands were sold before, and let that be next Friday hoping that we clear our conscience in front of God and his prophet and let everyone, after they finish the sacred ritual, gather for a peaceful demonstration to express their rejection of the agreement," al-Sadr wrote in the letter.
Sheikh Hazem al-Araji, a senior member of the Sadrist movement, told CNN the letter was read in Baghdad, Kufa, Amara, Nasiriya and other Iraqi cities before thousands of worshippers who attended Friday prayer services.
The United States and Iraq have been negotiating a proposed status of forces agreement for months.
The pact, which would set the terms for U.S. troops in Iraq after the U.N. mandate on their presence expires at the end of this year, has been controversial among many Iraqi officials.
Many say they will oppose any deal that hints of compromising the country's sovereignty.
Muslim cleric Grand Ayatollah Ali al-Sistani said in a statement on his official Web site this week that he will "forbid any stance that targets the sovereignty of Iraq no matter how small it is."