BAGHDAD (AP) - At least 13 people were stabbed Thursday in Baghdad´s Tahrir Square, the epicenter of Iraq´s protest movement, security and medical officials said, stoking fears of infiltration by unknown groups among anti-government demonstrators.

Parliament was scheduled to meet Thursday to amend laws governing compensation to include victims of security operations and vote on changes to the structure of Iraq´s electoral commission, the body that oversees polls across the country, according to two lawmakers in attendance.

Over a dozen protesters were attacked with knives by late afternoon, just as demonstrators supportive of political parties and Iran-backed militias withdrew from Tahrir, three anti-government protesters and a witness said. There were no fatalities.

The protesters aligned with parties had marched to Tahrir earlier that day, mostly young men clad in black and waiving Iraqi flags. They chanted positive slogans in deference to Grand Ayatollah Ali al-Sistani, Iraq´s most powerful Shiite cleric, and stood conspicuous against the usual crowds of Tahrir protesters.

Al-Sistani has largely sided with protesters, calling for serious electoral reforms and recently withdrew support for the government of Adil Abdul-Mahdi, prompting his resignation.

At least 400 people have died since the leaderless uprising shook Iraq on Oct. 1, with thousands of Iraqis taking to the streets in Baghdad and the predominantly Shiite southern Iraq decrying corruption, poor services, lack of jobs and calling for an end to the political system that was imposed after the 2003 U.S. invasion.

Hundreds of people march inside Tahrir Square carrying national flags and chanting religious slogans in Baghdad, Iraq, Thursday, Dec. 5, 2019. Anti-government protesters say multiple people have suffered stab wounds in Baghdad's Tahrir Square, the epicenter of their movement, after political parties and Iran-backed militia groups briefly joined them, raising fears of infiltration by authorities. (AP Photo/Hadi Mizban)

Security forces dispersed crowds with live fire, tear gas and sonic bombs, leading to fatalities.

"The parties and militias agreed to bring people to Tahrir under the pretext of maintaining peaceful demonstrations," said Mustafa, a demonstrator who requested anonymity fearing reprisal.

Around 2:30 p.m., the new group of protesters withdrew and left the square. Immediately after, over a dozen people were stabbed, the officials said. Demonstrators camped out in the square said the number was higher.

The injured were treated inside makeshift medical centers in the square and four with serious wounds were taken to al-Kindi hospital, in east Baghdad, medical officials said.

One police official said six individuals were taken into custody following the attacks.

It was not clear who, if anyone from the withdrawing group of protesters was responsible for the attacks; all wore plain clothes making their affiliations visibly unclear. The perpetrators had blended into the crowds of protesters who have effectively taken up residence in the sprawling plaza, two protesters and a security official said.

But the incident has fueled paranoia among protesters, who are convinced that members of Iran-backed militia groups disguised as demonstrators are to blame for the violence: "They hate the demonstrators and most withdrew. Those who stayed attacked the peaceful protesters in Tahrir," said a protester who requested anonymity, fearing reprisal from the authorities.

"They were strangers, those who did this," the protester said.

Immediate measures were taken to clear the square of possible saboteurs. Checks were conducted in the infamous Turkish Restaurant, a 14-story Sadaam Hussein-era building that emerged as a landmark in the protests.

Iran-backed militias are suspected of being behind the targeted sniping of protesters from Baghdad rooftops early in the uprising. The government has said it is unaware of which groups were responsible for the crackdown and denied they had been acting on orders of the Iraqi state authorities.

Another protester who requested anonymity said the attacks, "might have been perpetrated by the parties or someone who wants to ignite problems with the parties."

Iraqi officials have repeatedly warned of infiltrators seeking to co-opt and sabotage the largely peaceful movement. In his weekly sermon last week, al-Sistani warned of enemies seeking to sow strife and called on protesters to cast away those with ill intentions.

The stabbing incidents on Thursday were not the first in Tahrir Square. On at least three occasions in late October and November, protesters said, similar incidents had occurred, though not on the same scale as Thursday´s attacks.

Omar, 21, a medical student, recounted how in early November someone dressed in a white laboratory coat - common for volunteers - had stabbed a colleague inside one of the treatment tents.

Checkpoints were established to inspect individuals for weapons at entrances to the square. Meanwhile, some protesters fearing their movements were being watched by possible informants began wearing surgical masks to conceal their identities in public.

"The square is already a tight-knit community," said Amira, a protester in Tahrir, who only gave her first name. "We know when there are people who are not among us here, we talk to each other, ask questions, take precautions."

Hundreds of people march inside Tahrir Square carrying national flags and chanting religious slogans in Baghdad, Iraq, Thursday, Dec. 5, 2019. Anti-government protesters say multiple people have suffered stab wounds in Baghdad's Tahrir Square, the epicenter of their movement, after political parties and Iran-backed militia groups briefly joined them, raising fears of infiltration by authorities. (AP Photo/Hadi Mizban)

Hundreds of people march inside Tahrir Square carrying national flags and chanting religious slogans in Baghdad, Iraq, Thursday, Dec. 5, 2019. Anti-government protesters say multiple people have suffered stab wounds in Baghdad's Tahrir Square, the epicenter of their movement, after political parties and Iran-backed militia groups briefly joined them, raising fears of infiltration by authorities. (AP Photo/Hadi Mizban)

Hundreds of people march inside Tahrir Square carrying national flags and chanting religious slogans in Baghdad, Iraq, Thursday, Dec. 5, 2019. Anti-government protesters at least 15 people have suffered stab wounds in Baghdad's Tahrir Square, the epicenter of their movement, after political parties and Iran-backed militia groups briefly joined them, raising fears of infiltration by authorities. (AP Photo/Hadi Mizban)

Hundreds of people march inside Tahrir Square carrying national flags and chanting religious slogans in Baghdad, Iraq, Thursday, Dec. 5, 2019. Anti-government protesters at least 15 people have suffered stab wounds in Baghdad's Tahrir Square, the epicenter of their movement, after political parties and Iran-backed militia groups briefly joined them, raising fears of infiltration by authorities. (AP Photo/Hadi Mizban)