Thousands of Iraqis defied warnings from authorities and rallied in the heart of Baghdad on Friday, renewing pressure on the government to carry out reforms targeting corruption and sectarianism.

Although protests over the past year have resulted in repeated promises of reform, little in the way of concrete progress has been made, as those in a position to effect change benefit from the existing system.

Populist Shiite cleric Moqtada al-Sadr, who had called for the demonstration, made a brief appearance at the rally in Tahrir Square, which was packed with flag-waving protesters.

Protesters fought to get closer for a glimpse of the cleric but he quickly departed, and his remarks were instead read out by another speaker.

Sadr called for measures including the dismissal and trial of corrupt officials, an end to sectarian and political party quotas through which positions are shared out, and the formation of a government of technocrats, according to the remarks.

He has organised repeated protests calling for reforms, during which demonstrators have on several occasions breached Baghdad's Green Zone, a fortified area that is home to key government institutions and foreign embassies.

Speakers led the protesters in chanting slogans including: "Yes, yes to reform," "No, no to sectarianism" and "No, no to corruption."

The rally went ahead despite a warning from the government that it could distract security forces from the war against the Islamic State group.

A statement from the Joint Operations Command termed it "unauthorised" and said anyone who appeared with arms would be treated as a "terrorist threat."

The Friday demonstration, which started to wind down after Sadr's demands were read, was the first to take place in weeks, as the cleric called for a break in protests during Ramadan, when observant Muslims fast from dawn to dusk.

- 'Demanding our rights' -

Security forces fired tear gas to disperse demonstrators at previous protests, but this was one was much more tightly stewarded by organisers.

Protesters were repeatedly searched by men posted at makeshift checkpoints on the way to the demonstration, and strands of barbed wire kept them away from the bridge across the Tigris River that they had previously used to reach the Green Zone.

The route to the bridge was also blocked by slabs of heavy concrete blast wall and defended by riot police.

Internet connections were cut during the demonstration, but restored later in the morning.

"We are tired (of) corruption. Corruption is killing us," Mohamed al-Daradji, an activist and film-maker, said in a speech at the protest.

"These people (who) came after 2003... they failed. They failed! They didn't do anything," Daradji said, referring to politicians who came to power after the US-led overthrow of dictator Saddam Hussein.

Asked why he came to protest, Abu Mushtaq al-Awadi said: "We have rights, and we are demanding our rights."

Awadi said he wants an end to sectarian and party quotas, the trial of corrupt officials, the return of stolen money to the Iraqi people and the formation of a government of technocrats.

Sadr has previously called for a technocratic government to replace the current party-affiliated ministers -- a measure proposed by Prime Minister Haider al-Abadi -- but parliament has repeatedly failed to approve new ministers.

Abadi first called for a cabinet including technocrats in February, but has faced significant opposition from powerful political forces that rely on control of ministries for patronage and funds.

Some of Abadi's cabinet nominees were finally approved by parliament in April, but in a blow to the premier, a court later scrapped the session, which some disruptive lawmakers were barred from attending.