The protesters, who had gathered outside the heavily fortified district housing government buildings and many foreign embassies, crossed a bridge over the Tigris River chanting, ‘The cowards ran away!’ in apparent reference to departing lawmakers.
There were no reports of clashes with security forces. But special forces personnel from Iraq's army were dispatched with armoured vehicles to protect sensitive sites, two security officials said. No curfew has been imposed, they said.
A United Nations spokesman and four Western diplomats based inside the Green Zone said their compounds were locked down, denying local reports that staff were being evacuated.
An online video showed protesters attacking a white, armoured SUV with sticks and other objects. In a separate video, they beat a man in a grey suit.
A Kurdish peshmerga guard at a checkpoint said the protesters surged in after security forces pulled back from an external checkpoint in an unsuccessful effort to secure the parliament building. They had not been searched before entering the Green Zone, he said.
About ten members of the armed group loyal to Sadr were checking protesters cursorily as government security forces who usually conduct careful searches with bomb-sniffing dogs stood by the side, the witness said.
Local television broadcast footage from inside the parliament building showing hundreds of protesters dancing, waving Iraqi flags and chanting pro-Sadr slogans. Some appeared to be breaking furniture.
Rudaw TV showed them chanting and taking pictures of themselves inside the main chamber where moments earlier lawmakers had been meeting.
Thousands more protesters remained at the gates of the district chanting ‘Peaceful, peaceful!’. Some were standing on top of concrete blast walls that form the outer barrier to the Green Zone.
Supporters of Sadr, whose fighters once controlled swathes of Baghdad and helped defend the capital from Islamic State, have been demonstrating in Baghdad for weeks, responding to their leader's call to put pressure on Prime Minister Haider al-Abadi to follow through on months-old promises of reform.
Political parties have resisted efforts by Abadi to replace some ministers - chosen to balance Iraq's divisions along party, ethnic and sectarian lines - with technocrats in order to combat corruption. He has warned that any delay to the vote could hamper the war against Islamic State, which controls vast swathes of northern and western Iraq.