The violence was the most serious during a summer of unrest in Iraq, which has been without a government for the better part of a year and captive to escalating feuds between political factions, including followers of the cleric, Moqtada al-Sadr, and rival Shiite groups that are backed by Iran.

Sadr’s followers stormed the palace Monday after he announced his “final” retirement from politics — a threat he has made before, during years in the public eye, but one that could have more serious consequences in the charged political climate, and with the country ruled by a caretaker government.

“You are free of me,” Sadr told his supporters in a resignation message posted Monday afternoon on Twitter.

The fallout was immediate. Sadr’s supporters, who had been holding a sit-in inside the Green Zone, where government offices and diplomatic missions are located, scaled the gates of the palace and paraded through its ornate halls, in scenes shared on social media. Soon afterward, sounds of live ammunition echoed in the capital, as security forces descended on the protesters.

Elsewhere in Iraq, Sadr’s supporters blocked roads and government buildings, including in Basra, to the south. The U.N. mission in Iraq called the developments an “extremely dangerous escalation” and implored protesters to withdraw from the Green Zone.

“Iraqis cannot be held hostage to an unpredictable and untenable situation. The very survival of the State is at stake,” the mission said in a statement.

Iraq’s political dysfunction — a feature of civic life since the U.S. invasion nearly two decades ago entrenched a sectarian, kleptocratic order — entered its latest phase in October, when Sadr won the largest number of seats in parliament but failed to form a government. After months of political paralysis, Sadr withdrew his lawmakers from the legislature in June and sent his followers to occupy the parliament.

A rival political bloc, comprising Shiite groups backed by Iran, has also held protests and sit-ins in the Green Zone, raising fears of a confrontation. In the background of the political infighting, Iraqis have suffered mightily, as state institutions, from schools to hospitals, deteriorate without government support.

Sadr, a populist who has opposed both U.S. and Iranian influence in Iraq, has called for early elections, as well as the barring of political figures who served after the U.S. invasion from working in government.

The reasons for his latest political gambit were unclear, but it came on the same day an aging cleric who was considered a supporter of Sadr and his family announced his own retirement, in a statement that contained several digs at Sadr.

The statement by Grand Ayatollah Kadhim Husayni al-Haeri, who lives in Iran, called on his followers to support Iran’s supreme leader — rather than Iraq-based Shiite clerics — and also criticized Sadr, without naming him, suggesting he lacked the “conditions required” for leadership.

The statement had a “big impact” on Sadr, who probably thought that his Iran-backed Shiite rivals were behind the cleric’s retirement, said Ali Al-Mayali, an Iraqi political analyst. Those rivals, he said, had rejected Sadr’s attempts to form a government.

“Sadrists since the beginning have been hinting at civil disobedience as their last choice. I believe Sadr’s tweet … is the green light for the civil disobedience as his last step” against his Shiite rivals, Mayali said.

By nightfall, there were unconfirmed reports of armed attacks on installations used by Iran-backed Shiite militias across the country, including in Basra.

Health officials on Monday did not identify the victims of the violence in Baghdad but said some had been shot in the chest or stomach. A statement Monday night by Iraq’s caretaker prime minister, Mustafa al-Kadhimi, said the use of live ammunition by security forces was “strictly prohibited,” and he called for the protection of protesters.

Fahim reported from Istanbul.