Young demonstrators have thronged Baghdad and the Shiite-majority south since October, accusing the entrenched political elite of corruption and incompetence.

Last week, they brought down embattled prime minister Adel Abdel Mahdi, who resigned after top Shiite cleric Grand Ayatollah Ali Sistani intervened following a crackdown on protesters that has left around 430 people dead.

On Friday, Sistani said the Shiite religious leadership, or "marjaiyah," was not involved in talks on a new PM.

"The marjaiyah is not party to any discussions on this and has no role in any way whatsoever," he said, in a sermon read by his representative in the shrine city of Karbala.

He did, however, urge that a new premier be selected within the 15-day window outlined in the constitution and with no "foreign interference."

Iraq's main political blocs have been debating candidates for the premiership but have yet to name anyone.

Two key foreign officials have attended the talks, according to a senior political source -- Iran's pointman for Iraq Major General Qasem Soleimani and Mohammad Kawtharany, a leading power-broker from Lebanon's Hezbollah, a powerful Shiite movement.

Iran in particular wields tremendous sway in Iraqi political and military circles, especially within the Hashed al-Shaabi, a security force which has largely been integrated into the state.

David Schenker, the top US diplomat for the Middle East, on Friday slammed Soleimani's presence in Baghdad as "unorthodox" and a "huge violation" of Iraq's sovereignty.

Washington also announced sanctions against three Iran-linked Iraqi militia leaders -- Qais al-Khazali, Laith al-Khazali, and Hussein Falil Aziz al-Lami, also known as Abu Zeinab al-Lami.

The US accused them of assisting the crackdown on demonstrations. The measures would seize any assets they have in the US and keep them from travelling there.

Al-Lami is the head of the Hashed's security and Qais al-Khazali, who was already blacklisted, leads one of its most prominent factions, Asaib Ahl al-Haq.

- Newcomers at protests -

Protesters on the streets have publicly rejected what they say is Iran's overreach and have vented their anger against its diplomatic missions.

They were rattled on Thursday by the sudden arrival of several thousand Hashed supporters in Baghdad's Tahrir (Liberation) Square, the epicentre of the protests.

The Hashed began publicly supporting the protests after Sistani's call but demonstrators have expressed strong doubts about the Hashed's support, saying Thursday's display was an attempt to "ruin" their non-partisan rallies.

Apprehensive about a repetition on Friday, protesters erected new checkpoints around the square overnight and searched rucksacks of young demonstrators.

There were more newcomers on Friday, this time hundreds of clerics and officials from Shiite shrines in Baghdad, Karbala, Najaf and other cities.

With robed clerics in white turbans leading the way, a procession circled through Tahrir, carrying signs reading, "The marjaiyah is our support!"

"The sheikhs and clerics from Karbala took part in these protests, as well as convoys from the religious shrines," said Fadel Oz, an official from the revered Karbala shrine.

"This is in support of our brothers protesting in Tahrir."

One protester told AFP their presence made it less likely the rallies would descend into clashes.

"The religious shrines' involvement grants legitimacy and numbers to the protests," said Thaer Istayfi, 41.

- 'Not good enough!' -

Tahrir has become a melting pot of Iraqi society, occupied day and night by thousands of demonstrators angry with the political system in place since the aftermath of the US-led invasion of 2003 and Iran's role in propping it up.

Their public criticism of leading Iranian figures, including Soleimani, has broken a taboo and some among the protesters fear there will payback.

They worry they could be kidnapped or worse if they are identified as having opposed Iran or its allies, or simply for taking part in anti-government rallies.

Earlier this week, the bruised body of 19-year-old Zahra Ali was left outside her family home after she went missing, her father said.

And on Friday, the relatives of Zeid al-Khafaji, a photographer who had become well-known in the square, said he too had been kidnapped.

They said the 22-year-old had been snatched from outside his home by unidentified men in SUVs as he was returning from Tahrir.

Amnesty International said the reports of his abduction were "alarming."

"Local authorities have denied knowledge of the incident or his whereabouts. That is not good enough!" the rights group said.