Iraqi security forces fired tear gas and rubber-coated bullets at thousands of supporters of an influential Shia leader demonstrating near Baghdad's Green Zone to press for electoral reform.

Four protesters and one police officer were killed during Saturday's protests, Baghdad's governor told Al Jazeera.

Police and hospital officials said hundreds of protesers and seven other policemen were also injured in clashes.

The Associated Press news agency quoted hospital officials as saying that the officer died of a bullet wound.

Muqtada al-Sadr, the Shia leader, accused the elections commission of being corrupt and called for the commission's members to be changed, according to a statement from his office.

Rising to his call to protest, demonstrators had gathered near the Green Zone - a cluster of embassies and government buildings - to demand an overhaul of the commission that supervises elections before a provincial vote due in September.

Riot police fired tear gas when the crowd tried to move towards the zone, which also houses international organisations and the homes of prominent politicians.

Shots rang out in central Baghdad as security forces used live fire and tear gas to disperse the crowds.

"We will not give in to threats," said the head of the electoral commission, Serbat Mustafa, in an interview with a local Iraqi television channel Saturday afternoon.

Mustafa said he would not offer his resignation and accused Sadr of using the commission as a political "scapegoat."

Sadr has been a vocal critic of Prime Minister Haider al-Abadi, and last year protests that included many of his followers breached the highly fortified Green Zone twice.

An Associated Press news agency team at the scene witnessed ambulances rushing away protesters suffering from breathing difficulties.

The protest organisers said about two dozen demonstrators had choked on the gas.

Live TV footage showed young men running away as white smoke filled Tahrir Square in downtown Baghdad.

Sadr's supporters stormed the Green Zone last year after violent clashes with security forces.

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

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