At least seven protesters have been killed in Baghdad in an attack by unidentified men near the Iraqi capital's main protest camp, according to reports citing medical and security sources.

Gunmen on pick-up trucks late on Friday attacked a large building near the al-Sinak bridge where anti-government protesters had been camped out for weeks, witnesses told the AFP news agency.

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The attackers forced the protesters out of the building and live rounds could be heard after the altercation.

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

The attacks come one day after a string of suspicious stabbing incidents left at least 13 wounded in Baghdad's Tahrir Square, the epicentre of Iraq's leaderless weeks-long protest movement.

Al Jazeera's Simona Foltyn, reporting from Baghdad, said the reported violence marked "a very unusual escalation".

"What we've seen so far is casualties occurring because of direct confrontations between protesters and security forces near the front lines."

More than 400 protesters have been killed and nearly 20,000 wounded since anti-government demonstrations erupted on October 1.

Mass protests

Street protests have roiled Iraq since early October, with demonstrators calling for the resignation of the government, the dissolution of Parliament and an overhaul of the country's political system, which has been in place since the 2003 US-led invasion of Iraq.

The demonstrations have turned violent amid accusations from rights groups that Iraqi security members used excessive force against protesters.

On Friday, Iraq's top Shia leader Ali al-Sistani voiced support for the protests, calling them a "tool of pressure to affect genuine reforms" in the country.

"What matters most is that they [the protests] should not be dragged into acts of violence, chaos and sabotage," according to a mosque sermon read on his behalf in the southern city of Karbala.

Iraqi authorities have repeatedly accused "outlaws" of taking advantage of peaceful protests to attack demonstrators and security forces, and of vandalising public and private property.
      
On Sunday, Parliament accepted Prime Minister Adel Abdul Mahdi's resignation, bowing to a key demand of protesters.

Abdul Mahdi and his government are now serving in a caretaker status until a new cabinet is formed.