BAGHDAD — A wave of fresh unrest rippled across Iraq on Thursday as security forces clashed with demonstrators in Baghdad and cities in the south, leaving more than two dozen protesters dead and a diplomatic mission burned, and prompting Iraq’s government to form new “crisis cells” to manage the unfolding turmoil.

The latest surge of violence — which included an attack on the Iranian Consulate in Najaf, a city sacred to Shiite Muslims — underscored the deep challenges for authorities after nearly two months of anti-government protests over a high unemployment rate, corruption and poor government services in this oil-rich ­nation.

It also draws Iran, governed by a Shiite theocracy, deeper into the unrest. Iran is a major backer of the Iraqi government and holds powerful sway over local Shiite militias.

Protesters, meanwhile, intensified pressure on Iraq’s embattled leadership with bold moves to occupy bridges and strategic roads.

In Baghdad, the capital, marchers tried to hold bridges leading to the fortified Green Zone, the site of many government offices. In Najaf, blockades of burning tires cut off parts of the city.

The standoffs turned deadly in Nasiriyah, about 160 miles south of Baghdad, where Iraqi security forces shot dead at least 22 protesters who had gathered on a bridge early Thursday, a local medical official said.

He described the scene at Nasiriyah’s main hospital as “tragic,” with women wailing over the bodies of dead family members.

Protesters also blocked a major highway and surrounded the local police station, according to a local activist, 31-year-old Ali al-Ghizzi.

“The scenes from Nasiriyah . . . more closely resemble a war zone than city streets and bridges,” said Lynn Maalouf, Middle East research director at Amnesty International.

The complete death toll across Iraq was unclear. The Reuters news agency, citing medical officials, said at least 35 people were killed.

In Najaf, the government ordered a citywide curfew after demonstrators torched Iran’s consulate overnight. Consular staff members were evacuated ahead of time, Iranian media reported, but the action drew a stern rebuke Thursday from Iranian Foreign Ministry spokesman Abbas Mousavi, who demanded that the perpetrators be brought to justice and reminded the Iraqi government of its responsibility to protect diplomatic enclaves.

The attack follows one on the Iranian Consulate in the city of Karbala on Nov. 4.

Iraq’s government set up “crisis cells” in several provinces to manage the crisis. According to a military statement, the cells “would be led by provincial governors but include military leaders who would take charge of local security forces,” Reuters reported.

Protesters in Najaf, who have been blocking roads and demonstrating for the past few days, say they turned on the consulate because their demands were not being heard. In Baghdad on Thursday, four more demonstrators were killed as they clashed with security forces on a strategic bridge near the main protest site.

“For a month we’ve been protesting, but the government treated us like we don’t even exist,” said Yasser Mahmoud, a local activist, who said that protests in Najaf had been peaceful until now.

“We went for the consulate because now we are aware who is the reason behind all that we are going through,” he said. “Iran controls all these Islamic parties and their armed militias in the streets that have made our life like hell — they are controlling the economy of the city and everything goes to them. Meanwhile, we can’t even get a job.”

The protests, calling for broad changes to the political system, have had a distinctly anti-Iranian aspect to them, especially in the country’s largely Shiite Muslim south.

Najaf has been a focal point of Iranian influence because of its importance to the Shiite branch of Islam. It is home to the main seminaries and the most powerful Shiite clerics — including Grand Ayatollah Ali Sistani — and is a focus for Shiite pilgrimage, with hundreds of thousands visiting from Iran alone.

Militia leaders close to Iran issued a statement Thursday saying that Sistani, the spiritual leader for much of Iraq’s Shiite majority, was in danger because of the protests and vowed to protect him.

Sistani, who is Iranian, has sought to curb the Shiite theocracy’s influence in Iraq and has backed the demonstrators’ calls for reform.

“We will cut the hands of anyone who tries to touch him,” said Abu Mahdi al-Muhandes, commander of the Popular Mobilization Forces, an umbrella coalition of Shiite militias with close ties to Iran.

Qais al-Khazali, another powerful militia leader, traveled to Najaf, where he issued a warning to protesters.

“Whoever wants to test us is welcome to try,” he said.

The remarks raised fears that groups loyal to Iran were using the protests in the holy city — which is also home to the shrine of Imam Ali, one of the most revered figures in Shiite Islam — as a pretext to crack down on demonstrators. A militia backed by Sistani that operates independently of Iran rejected the offers to help protect the ayatollah.

Residents of Najaf mocked the pro-Iran militia leaders’ comments on social media, saying that their grievances are with Iran, not Sistani, who has opposed Iranian influence.

“These statements are a joke. Everybody knows that no one will get close to Sistani,” said Emad al-Kaabi, 26, an activist in Najaf.

Protesters pointed to the fact that the Iranian Consulate is located far from the old city, where Sistani lives.

Kaabi added that the militia leaders’ statements were just an excuse to “deploy their forces in the streets and take revenge for the burning of the consulate.”

According to the Associated Press, at least one person was killed in the attack on the consulate and at least 35 were wounded.

At least 350 people have died in Iraq since protests erupted on Oct. 1, with daily battles in the heart of Baghdad as protesters attempt to gain control of the key downtown bridges leading to the seat of government.

Security forces have confronted crowds with live ammunition, rubber bullets and tear gas, often with fatal results.

Iran was convulsed by protests over gasoline price increases this month that the Interior Ministry said Wednesday involved about 200,000 people nationwide with up to 7,000 detained, according to local media.

In a speech, Ayatollah Ali Khamenei, Iran’s supreme leader, blamed a conspiracy involving the United States for the unrest.

Cunningham reported from Istanbul. Paul Schemm in Dubai and Brian Murphy in Washington contributed to this report.