Iraqi Prime Minister Haider al-Abadi had called on the demonstrators, most of them supporters of cleric Moqtada al-Sadr, to stay home and security forces deployed to block their way to the Green Zone, but the protesters were undeterred.
Demonstrators pushed past security forces at Tahrir Square, breached a barbed wire barrier and attempted to pull down slabs of heavy concrete blast wall blocking Jumhuriyah Bridge, which leads toward the Green Zone, where Iraq's main government institutions are located.
Many of the protesters, most of them supporters of powerful Najaf-based cleric Sadr, had come equipped with gas masks or surgical masks.
"Those with masks, go this way to pick up the injured," one protester instructed his comrades after the first canisters were fired at the crowd.
Demonstrators forced open a gate to the Green Zone and stormed the premier's office last Friday before being driven away by security forces.
Abadi had sought to head off a repeat this week, calling on yesterday for protesters to postpone their demonstration, as security forces are busy fighting to retake the city of Fallujah from the Islamic State group.
Saif, a 23-year-old protester who was recovering from the effects of tear gas, said he had two brothers who were killed fighting IS.
"I hope our forces finish the job in Fallujah; I wish them well, of course," Saif said.
The protest eventually wrapped up with only minor injuries reported, mostly cases of suffocation.
