The assailants briefly ousted demonstrators from a building they had occupied for weeks in Baghdad, despite the presence of security forces nearby who did not intervene.

The panicked protesters rushed out into the street, sending out calls through social media for people to come to their main gathering place in Tahrir Square.

Before dawn on Saturday, hundreds had arrived.

"I came after the incident and there were tons of people in Tahrir and by Al-Sinek," a nearby bridge, one demonstrator told AFP, adding he was shocked by the lax security measures.

"The police were there but didn't even search me," he said.

Many of the new arrivals were suspected to be members of Saraya al-Salam (Peace Brigades), the paramilitary group headed by Iraqi cleric Moqtada Sadr.

Sadr has backed the protests, but many demonstrators who were proud of their movement's relative political independence have been wary of his support.

A source within Saraya told AFP one of its own members had died in the overnight clash, and that more unarmed members had been sent to Tahrir "to protect protesters."

The demonstrator said he saw men in Tahrir carrying sticks and wearing cargo vests with makeshift fire bombs packed inside them.

Further south in Nasiriyah, the usual rallies swelled with crowds upset over the previous night's developments in Baghdad, an AFP correspondent said.

"We are coming in solidarity with Baghdad," one said.

Security forces were also deployed in Nasiriyah, where protests have continued despite an attempted crackdown last week that left more than two dozen dead.

In Diwaniyah, another protest hotspot, thousands turned out early on Saturday but security forces, too, spread across the streets in larger numbers.

The overnight bloodshed rattled protesters, who had feared a spiral into chaos after supporters of the Hashed al-Shaabi security force flooded Tahrir on Thursday.

They worried such a show of force could set up a confrontation between the Hashed and others in the square.

Around 440 people have died and 20,000 have been wounded since anti-government rallies erupted on October 1 in Baghdad and the Shiite-majority south.