Around 460 people have been killed and 25,000 wounded, most of them protesters, since anti-government rallies erupted on October 1 in Baghdad and the Shiite-majority south.

Since then demonstrators in the capital and southern cities have disappeared almost daily, in most cases taken from near their homes as they returned from protests.

"We strongly denounce the killings, abductions and attacks of all kinds that have been taking place," Grand Ayatollah Ali Sistani said in a sermon delivered by his representative in the shrine city of Kerbala during the weekly Friday prayer.

London-based rights group Amnesty International on Friday urged Baghdad to clamp down on what it called a "campaign of terror targeting protesters".

Protesters accuse pro-Iran armed factions of playing a role in the killings and abductions.

Sistani specifically mentioned an attack late last Friday, when unidentified gunmen stormed and torched a multi-storey building in Baghdad where protesters had camped out for weeks.

At least 20 protesters and four police officers were killed and about 80 demonstrators abducted, medical sources and witnesses said.

The revered Shiite cleric also denounced Thursday's lynching by demonstrators of a teenager accused of attacking protesters in Baghdad, calling it "an atrocious crime".

Such actions "reaffirm once again the importance of what the marjaiyah (Shiite religious leadership) has repeatedly called for and that is that all weapons must be placed under the control of the state", he said.

Sistani urged authorities to prosecute those responsible for the violence and called on protesters to keep their demonstrations peaceful.

Amnesty in its statements also called on Iraq's authorities "to step up to their responsibilities".

Iraq must "take immediate and effective action to put an end to a growing lethal campaign of harassment, intimidation, abductions and deliberate killings of activists and protesters," it said.

- 'Horrifying new stage' -

"The authorities' utter lack of action over the past weeks has paved the way for this horrifying new stage in what is clearly a full-on attempt to crush the protests in Iraq through instilling fear among the population," said Amnesty's Middle East Research director Lynn Maalouf.

She said government inaction indicated acquiescence and in some cases complicity in the "enforced disappearances, torture and unlawful killings of people who are on the streets to claim their human rights".

The mostly young protesters are angry at a government they see as corrupt, inefficient and under the sway of neighbouring Iran.

They have complained of unemployment and poor infrastructure including chronic power and water cuts, despite the country's vast oil wealth.

One in five Iraqis live in poverty and youth unemployment stands at a quarter, the World Bank says.

Demonstrators have also turned their wrath against Iran, which wields tremendous sway among Iraqi politicians and military figures, particularly the Hashed al-Shaabi paramilitary forces formed to battle the Islamic State group.

At dawn on Friday, sound bombs exploded near pro-Iranian targets in the southern city of Amara without causing any casualties, security sources said.

Two of them targeted the homes of leaders of the Asaib Ahl al-Haq, one of the most powerful groups within the Hashed, and a third went off near the house of a member of Ansar Allah, another component of the group.

Protests were also reported in several cities in southern Iraq as well as in Baghdad's Tahrir Square.

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