Security forces fired tear gas as thousands of protesters gathered in central Baghdad on Friday and attempted to head to the Green Zone, a fortified area they have breached twice.

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.

Some protesters, who are calling for a new government, gave olive branches and flowers to security personnel at Tahrir, but events quickly escalated, and the forces fired tear gas in an attempt to disperse the demonstration.

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.

They used tear gas, water cannons, sound bombs and a barrage of bullets largely fired into the air to disperse the protesters and harry them away from the Green Zone, killing at least two people and injuring dozens.

Abadi had sought to head off a repeat this week, calling on Thursday 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.

"But if those corrupt people in the Green Zone weren't there in the first place, there would be no Daesh and no war against Daesh," he said, using an Arabic acronym for IS.

The protest eventually wrapped up with only minor injuries reported, mostly cases of suffocation.

Demonstrators faced little resistance when they entered the Green Zone and overran parliament in late April, but the period of tolerance of such actions has ended.

Protests have been held almost every Friday for weeks by people demanding the current government be replaced with technocrats.

Abadi proposed that measure in February, but has faced opposition from powerful parties that rely on control of ministries for patronage and funds.

Sadr, a Najaf-based cleric who led an insurgency against US-led forces, has also demanded a technocratic government, encouraging his supporters to call for the change.