Iraqi security forces fired tear gas and rubber-coated bullets Saturday on protesters who attempted to march on Baghdad's fortified Green Zone to press demands for electoral reform, police said.

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.