The protesters on Friday were mostly supporters of the "Group of the Virtuous", a Shia group calling for the direct implementation of Islamic law in state affairs and for the establishment of a theocratic political system similar to Iran's.
"They want Iraq to split into many countries, and they want us to be their subjects," they chanted. "We will never accept a constitution written by the Jews."
Shaik Muhammad al-Yaaqubi, a self-proclaimed Ayat Allah who heads the group, accused Iraq’s occupiers of ignoring Shia rights.
"This law has lots of gaps that can bring evil. Whatever is said to justify this document ...is false," he said in a statement.
Iraq’s Shia have protested that the recently signed interim constitution gives minority groups like the Kurds too much influence over the country’s future.
Members of the US-appointed Governing Council signed the document on Monday, after weeks of wrangling. But most Shia members of the Council said they were unhappy with it and hoped to make changes.
"The religious authority was not consulted in this constitution, it's a pre-made dish cooked by the Americans and their puppets in the Governing Council"
Iraq's most influential Shia cleric, Ayat Allah Ali al-Sistani, issued a statement after the signing saying the interim constitution was deeply flawed.
The document will guide an Iraqi government due to take power on 30 June until a permanent constitution is drawn up next year.
At the Baghdad protest, young men waving Iraqi flags chanted slogans denouncing the United States and Israel.
More Islamic influence
"This constitution does not represent us. It is an attempt to stop the Islamists from taking power in this country. We denounce this constitution that was written by American hands," said Ahmad Said, a demonstrator.
Iraqi Shia clerics lead Friday
prayer on the street in Baghdad
"The religious authority was not consulted in this constitution, it's a pre-made dish cooked by the Americans and their puppets in the Governing Council. We are ready to sacrifice our lives to change this constitution," said Shaikh Hadi Waili, another protester.
Muqtada al-Sadr, a firebrand Shia cleric with a large following among the Shia urban poor in Baghdad, also denounced the constitution in his Friday sermon in Kufa, 160km south of Baghdad.
He urged for a new text to be written based on the Quran and what he said were Iraqi people's own ideas.
"I call on the Kurds to come closer to their Muslim brothers and to remember their Islamic identity, which is more important than their Kurdish identity," he said.