Basra, Iraq ? Shouting “no to America!” tens of thousands of Shiite Muslims took to the streets Thursday to protest a U.S.-backed formula for choosing Iraq’s new legislature.

The protest came as an aide to Iraq’s foremost Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, warned that he might issue a fatwa, or religious edict, rejecting a U.S.-backed government if his demands for direct elections are ignored.

The turnout in Basra, estimated by British soldiers at up to 30,000, was the biggest protest organized by Shiite clerics against the power transfer plan.

Al-Sistani and other clerics wield vast influence among Iraq’s Shiites, thought to make up about 60 percent of the country’s 25 million people.