The flash protest hinted at a new effort to delegitimise or intimidate anti-government protesters, and came after the apparent torture and death of a 19-year-old girl taking part in the rallies.

Youth-dominated mass rallies have rocked Iraq's capital and Shiite-majority south since October, slamming the entrenched political class as corrupt, incompetent and under the sway of neighbouring Iran.

Tehran holds vast influence among Iraq's ruling figures and military actors, chiefly the armed, Shiite-majority Hashed al-Shaabi network.

On Thursday, several thousand Hashed supporters marched into Baghdad's Tahrir Square, the epicentre of anti-government protests.

In an apparent show of force, they waved sticks, Iraqi flags and the Hashed's logo -- a symbol so far explicitly shunned by the vast majority of protesters.

Some of the newcomers carried portraits of Hashed fighters killed while battling jihadists and of the country's top Shiite cleric, Grand Ayatollah Ali Sistani.

The Hashed initially backed Iraq's government against the protests, but had dropped its support in line with a call from Sistani following a spree of violence against rallies.

Thursday's arrivals carried signs opposing "vandals", a reference to people who have damaged properties during rallies, but the protesters sensed something more threatening.

"They've ruined it," said one anti-government protester.

Another muttered: "It's going to get messy."

- Gruesome deaths -

"They came here to clear us out and end the protests," said Tamim, a 30-year-old demonstrator wearing a protective vest.

There were no confrontations on Thursday, but Harith Hasan of Carnegie's Middle East Center said the development could increase tensions.

"This could be the beginning of a competition or conflict to occupy the public space," he wrote on Twitter.

Toby Dodge, director of the Middle East Centre at the London School of Economics, said it appeared to be "a new tactic to 'crowd out' or close the space down available to the demonstrators".

Activists in Baghdad and elsewhere have already been subject to threats, kidnappings and even killings in what they say are attempts to keep them from demonstrations.

In one particularly gruesome case, the bruised body of 19-year-old Zahra Ali was left outside her family home late Monday, her father, Ali Salman, told AFP.

She had been missing since that morning.

"We had been distributing food and drink to protesters in Tahrir and we had never been threatened, but some people took pictures of us," Salman said.

"The court established she had suffered electrical shocks," he added.

It was unclear who had kidnapped her.

Baghdad authorities announced this week they were releasing 16 detained protesters, after Human Rights Watch accused the government of not doing enough to protect activists.

- 'Take a stance' -

Mass rallies have rocked Iraq's capital and south since early October, first denouncing government graft and a lack of jobs before moving to broader demands for deep-rooted regime change.

Nearly 430 people have been killed and 20,000 wounded since demonstrations erupted, according to an AFP toll compiled from medics, police and a national commission.

The victims' families have been demanding justice and many of them hit the streets of Iraq's southern Diwaniyah on Thursday.

They joined thousands of other protesters, mostly teachers and students taking part in a general strike, AFP's correspondent said.

"The authorities are putting off the issue of who killed our sons and brothers in the protests," said Assaad Malek, whose brother died in protest-related violence.

"They should take a tough stance and severely punish the officers and SWAT forces who killed my brother," he added.

A verdict for security force members accused of violence against protesters in Diwaniyah was scheduled for Thursday, but the session was indefinitely postponed.

On Sunday, an Iraqi court sentenced a police officer to death after convicting him of killing demonstrators, the first such sentence in the unrest.

In the southern hotspot of Nasiriyah, hundreds gathered at the main protest camp, joined by delegations from the province's powerful tribes.

Tribal dignitaries intervened last week to quell tensions between protesters and security forces after more than two dozen people were killed.

A bloody crackdown left several dozen dead in the south last week, ultimately leading to embattled prime minister Adel Abdel Mahdi's resignation at the weekend.

Parliament has given President Barham Saleh until December 17 to name a replacement.

On Thursday evening, it also passed a law to reform the country's electoral commission.

Iran's pointman on Iraq, Islamic Revolutionary Guard Corps commander Qasem Soleimani, has been in Baghdad to rally political forces around a new premier.