About 7,000 protesters, many of them women, marched in the streets of the southern city of Basra to express their anger at a violent crackdown on the protesters by Bahrain's Sunni monarchy.
"Gulf rulers are the slaves of America," read a placard carried by the crowd, while another proclaimed, "the hands of Bahrain's rulers are tainted with the blood of innocents."
The demonstrators marched toward the governorate headquarters in Basra and denounced the Saudi-led Gulf force send to put down the unrest.
They carried effigies of the kings of Bahrain and Saudi Arabia, each adorned with a garland of sandals as an insult.
"We are demonstrating today to support our brothers in Bahrain," said Mohammed Hassan, a 35-year-old taking part in Saturday's protest in Basra.
"We are waiting to see what our religious leaders say, and we are ready to go help our brothers in Bahrain."
International pressure is mounting on Bahrain to exercise restraint and ensure the safety of opposition leaders arrested after security forces quelled a month-old pro-democracy protest, as the king promised to press on with reforms.
More than 20,000 people had rallied across Iraq on Friday, and several thousand on Wednesday, in demonstrations organised by the radical Iraqi Shiite cleric Moqtada al-Sadr.
The demonstration in Basra, and a smaller protest of about 200 people in Baghdad, was organised by the Shiite Dawa party.