Most of the thousands of protesters gathered in the heart of the capital were supporters of populist cleric Moqtada al-Sadr, who is making a fresh push for change ahead of provincial elections due in September.
The protest started peacefully and several speakers addressed the large crowd in Tahrir Square before some of the demonstrators broke away and attempted to break through a security cordon guarding the main road to the Green Zone.
"The demonstrators tried to cross Jumhuriya bridge, the security forces fired tear gas to stop them but they insisted," a senior police official said.
Another police official said at least 11 protesters were wounded by rubber-coated bullets or tear gas canisters when the violence erupted, while dozens were also treated for breathing difficulties.
AFP correspondents saw several wounded protesters in the crowd and television footage showed at least two people with what appeared to be serious head injuries.
The demonstrators, who had gathered early on Saturday, received a de facto green light to escalate the protest in the shape of a statement from the Najaf-based Sadr.
"If you want to approach the gates of the Green Zone to affirm your demands and make them heard to those on the other side of the fence... you can," he said.
He encouraged the protesters to remain there until sunset but warned them against attempting to break into the fortified area.
Pressure on Abadi
His supporters stormed the Green Zone, which is home to the electoral commission and most of the country's other key institutions, on two occasions last year.
The protesters on Saturday were met with fierce resistance from the security forces and never made it across the Tigris River that runs between Tahrir Square and the Green Zone.
But Sadr, a mercurial Shiite cleric who once led a rebellion against US occupation but has more recently spearheaded an anti-corruption protest movement, urged Prime Minister Haider al-Abadi not to turn a deaf ear.
"I urge him to deliver those reforms immediately, listen to the voice of the people and remove the corrupt," the statement said.
A smaller group of protesters had already demonstrated near the Green Zone on Wednesday, while hundreds also gathered in several southern cities on Friday.
Their two main demands are for the members of the electoral commission to be replaced on the grounds that they are all affiliated to political parties and that the body supervising nationwide ballots was therefore anything but independent.
They also want electoral law to be amended to give a wider berth to smaller parties in the country's elected bodies.
The current seat allocation system for parliament was adopted before 2014 parliamentary polls, after small parties made significant gains in provincial elections a year earlier.
Last year's protest movement was halted when tens of thousands of forces launched Iraq's largest military operation in years four months ago to retake the city of Mosul from the Islamic State group.
The announcement last month that elections would take place in September however has brought the political agenda back to the fore and Sadr's movement announced it would up the pressure again.
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