Protesters had feared possible violence since Thursday, when thousands of supporters of the Hashed al-Shaabi force descended on Tahrir Square in a show of force.

Earlier Friday, the anti-government demonstrations had been relatively calm, with some saying the presence of religious clerics had kept the rallies from deteriorating into chaos.

But after night fell, armed men on pick-up trucks attacked a large building near the Al-Sinek bridge where anti-government protesters had been camped out for weeks, witnesses told AFP.

The gunmen forced the protesters out of the building and gunshots could be heard after the altercation.

State television said the building had been torched "by unidentified men".

Medics said dozens of other demonstrators were wounded.

One female medic at a field clinic nearby said she had treated at least five people for light stab wounds.

The new deaths bring the toll to more than 430 dead and nearly 20,000 wounded since anti-government demonstrations erupted on October 1.

The protesters had prided themselves on their relative independence and were rattled on Thursday by the sudden arrival of several thousand Hashed supporters in Tahrir Square.

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

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

It began publicly supporting the rallies, but demonstrators have expressed doubts those sentiments were genuine.

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