Iraqi security forces fired tear gas on Saturday at thousands of supporters of Shia cleric Moqtada al-Sadr demonstrating near the heavily fortified Green Zone - a cluster of embassies and government buildings - to press for reforms, witnesses said.

The protest organisers said about two dozen demonstrators had choked on the gas but no one was seriously injured or taken to hospital. Live TV footage showed young men running away as white smoke filled Tahrir Square in downtown Baghdad.

Thousands had gathered in the square to demand an overhaul of the commission that supervises elections ahead of a provincial vote due in September.

Riot police fired tear gas when they tried to move toward the Green Zone which also houses international organisations. Sadr's supporters stormed this district last year after violent clashes with security forces.

Sadr suspects that members of the electoral commission are loyal to his Shia rival, former Prime Minister Nuri al-Maliki, one of the closest allies of Iran in Iraq.

Iraqi Prime Minister Haider al-Abadi called on the demonstrators to remain peaceful and to "abide by the law".

Sadr is openly hostile to American presence and policies in the Middle East and, at the same time, he has a troubled relationship with Iraqi political groups allied with Iran.