Supporters of Iraq's powerful Shiite Muslim cleric Moqtada Sadr gathered for Friday prayers ahead of a counter-rally by their opponents later in the day.
The opposing demonstrations are the latest turn in a political standoff which has so far remained peaceful in the war-scarred country.
Thousands of supporters of Mr. Sadr, who once led a militia against American and Iraqi government forces, gathered for the traditional weekly prayers near parliament inside the normally secure Green Zone.
A week earlier, Mr. Sadr had called out tens of thousands of his followers for prayers in the area, home to government and diplomatic buildings.
For nearly two weeks, his supporters have held a sit-in, first inside the legislature, and more recently on its grounds.
Their protest, triggered by a rival bloc's pick for prime minister, reflects months of failed negotiations by Iran's political forces to form a government after October elections.
Outside parliament Umm Hussein, a Sadr supporter in her 50s, said she was there to protest "the regime that for 20 years has done nothing for the people, except plunder and steal public money".
On Wednesday, Mr. Sadr demanded the judiciary dissolve parliament by the end of next week, as part of his call for new elections.
His rivals in the pro-Iran Coordination Framework plan to demonstrate at around 5:00 pm (1400 GMT) on a road leading towards the normally secure Green Zone where parliament is located.
A statement from the alliance said it will demonstrate for the "formation of a new government" that would provide public services and solutions to power outages and water shortages.
The Framework had initially said they were conditionally open to new elections.
Two days after Mr. Sadr supporters stormed the Green Zone and entered parliament on July 30, thousands of Coordination Framework backers held a counter-protest on a road leading to the Green Zone.
Police fired water cannons to prevent them from entering the area, and they dispersed after about two hours.