Moqtada Al Sadr called on Thursday for an urgent session of Iraq's parliament to discuss the situation in Basra, after protests in the southern city turned violent.
A curfew was planned for 3:00pm local time in a desperate attempt to quell the demonstrations, but it was lifted minutes before it was due to start. An Interior Ministry spokesman cited "intelligence reports of possible attacks on government offices" as the reason behind the failed curfew.
The violent clashes followed a call earlier on Wednesday by the UN envoy to Iraq for "calm".
Jan Kubis also urged the authorities "to avoid using disproportionate, lethal force against the demonstrators" and asked the government to "investigate and hold accountable those responsible for the outbreak of violence [while also doing] its utmost to respond to the people's rightful demands of clean water and electricity supplies".
Nine civilians have been killed since the start of the month in clashes between residents and security forces.
On Wednesday, demonstrators - angry over electricity outages, water contamination, a lack of jobs and proper government services - took the streets again. One person was killed and 25 injured, some seriously, as Iraqi security forces opened fire on protesters.
In a televised speech Mr Al Sadr, a Shiite cleric whose allies took the most seats but not a majority in May elections, said Prime Minister Haider Al Abadi and other officials should either attend the session or resign. Iraq's feuding factions have yet to form a new government.
The session, he added, should be held no later than Sunday. Officials in Baghdad have been locked in a squabble over the formation of the next government.
On Wednesday an Agence France-Presse journalist reported gunshots and tear gas were fired by security personnel directly at demonstrators, who had gathered in their thousands outside the regional government headquarters.
The measures failed to disperse protesters, who responded by hurling Molotov cocktails and letting off fireworks at the security forces.
Security forces later stopped firing in the direction of protesters and instead fired shots in the air and used tear gas.
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Reinforcements had been sent to Basra and concrete blocks erected to protect the government building.
Basra province and other southern cities have been the focus of angry anti-government demonstrations that have rocked Iraq since early July.
Residents are particularly angry over pollution of the local water supply, which has put 20,000 people in hospital.
On Tuesday six demonstrators were killed and more than 20 wounded during the bloodiest day of clashes with security officials, a local official and medics said.
The authorities said that 30 security personnel were also wounded in the violence "by grenades and incendiary objects".
On Tuesday, Prime Minister Al Abadi said he had ordered "no real bullets... to be fired, in the direction of protesters or in the air".
Mr Al Sadr said in a tweet ahead of the latest clashes that "vandals infiltrated" the protests.
The authorities have pledged to take measures to put an end to the health crisis that has ravaged the oil-rich province of Basra.
Mr Al Abadi announced overnight that he had met lawmakers from Basra, who are in Baghdad for the first parliamentary session since the elections.
He again indicated that water pollution would be addressed, without specifying any measures.
In July, the government announced a multi-billion dollar emergency plan for southern Iraq, to revive infrastructure and services.
But protesters are wary of promises made by the outgoing government, as negotiations drag on over the formation of the next administration.