Deaths -- including that of a 14-year-old boy -- were reported in at least eight cities, including Fallujah, Mosul and Tikrit, The Washington Post reported Sunday.
Witnesses said security forces fired live bullets and water cannons at demonstrators Friday and described the detentions as an operation to frighten intellectuals who could influence public opinion.
Four journalists detained in Baghdad said they were blindfolded, handcuffed, beaten and threatened with execution before being released Saturday.
"It was like they were dealing with a bunch of al-Qaida operatives, not a group of journalists," said Hussan al-Ssairi, one of the journalists detained. "Yesterday was like a test, like a picture of the new democracy in Iraq."
Ssairi said he and his colleagues were sitting at an outdoor restaurant about 2 miles from Baghdad's Tahrir Square, where the demonstrations were focused. He said security forces arrived in two Humvees and started beating a group of people, including a journalist.
"We said, 'What are you doing -- we're journalists!'" said journalist Hadi al-Mahdi. "And they said '(expletive) journalism.'"
Unlike the anti-government protests in the Middle East and North Africa, demonstrators in Iraq were calling for reform and an end to corruption, rather than the overthrow of the government.