Iraq's top cleric called on politicians to slash their benefits and improve public services on Saturday, a day after thousands of protesters took to the streets in a nationwide "Day of Rage".

A human rights group, meanwhile, said investigations had to be opened into the deaths of demonstrators who rallied against high levels of corruption and unemployment as well as poor public services.

In Baghdad, traffic was once again allowed to pass through the capital's Tahrir Square where 5,000 demonstrators had gathered, the biggest of at least 17 separate protests across Iraq.

While the majority of protests were mostly peaceful, clashes with police left 15 demonstrators dead and more than 130 injured, according to an AFP tally based on official sources. Four government buildings were set ablaze and one provincial governor resigned.

The top Shia Muslim religious authority, Grand Ayatollah Ali Al-Sistani, said that the government had made progress on improving power supplies, providing food for the needy, creating jobs and combatting corruption.

But he also called on Iraq's leaders to "cancel unacceptable benefits" given to current and former politicians, and said they must "not invent unnecessary government positions that cost Iraq money".

Sistani, who is based in the central shrine city of Najaf and rarely wades into politics, warned that the "current way of managing the state will lead to delays in taking radical solutions for people's problems."

His remarks came after New York-based Human Rights Watch called on Iraqi authorities to open inquiries into all deaths as well as any illegal use of force by security forces.

"Any unlawful use of force... should lead to the prosecution of those responsible," the New York-based watchdog said in a statement.

Despite most traffic curbs being lifted on the capital, concrete blast walls remained stationed on Jumhuriyah bridge, which connects Tahrir Square to Baghdad's heavily-fortified Green Zone, home to the US embassy and parliament.

On Friday, security forces at the square used water cannons and tear gas to disperse angry demonstrators, who had thrown stones, shoes and plastic bottles at riot police and soldiers blocking off the bridge. An interior ministry official said 15 people were wounded.

North of the capital, clashes between security forces and demonstrators in the cities of Mosul and Tikrit each left five people dead, while two died in the town of Hawija.

Two demonstrators were killed in Samarra and a 15-year-old boy died in the mostly Kurdish town of Kalar in central Diyala province.

Rallies in Iraq have called for improved public services, more jobs and less corruption, and some for broader political reforms.

Rated the fourth-most corrupt country in the world by Transparency International, Iraq suffers from poor electricity and water provision, as well as high unemployment, nearly eight years after the US-led invasion.

In a bid to head off protests, Iraq has cut politicians' pay, increased food aid for the needy and delayed a planned law that would have raised import tariffs with knock-on effects on the price of basic goods.