BAGHDAD — A day after more than 40 protesters were killed by security forces, Iraq’s prime minister announced Friday that he would submit his resignation to parliament, a step that carried uncertainty for the entire government and stirred fears of a possible political crisis.

The move by Adel Abdul-Mahdi came 13 months after he took office and followed calls by Iraq’s top Shiite cleric for lawmakers to withdraw support. At least four protesters were killed in the hours after the announcement in continuing violence in Baghdad and southern Iraq.

Word of the planned resignation triggered celebrations by anti-government protesters who have been camped out for nearly two months in Baghdad’s Tahrir Square. Young men and women broke into song and dance under the sparkle of fireworks crackling from every corner of the plaza, the epicenter of their leaderless protest movement, which seeks an end to sectarian government and election and anti-corruption reforms.

But amid the mirth, protesters said Abdul Mahdi’s decision was a single victory in the long and difficult war aimed at dismantling the post-2003 political system, a common refrain among demonstrators.

“The political system will replace him with someone exactly the same,” said Taif, a 39-year-old protester, as jubilant demonstrators waved flags behind. “Until this sick system is destroyed, we won’t leave.”

On the street near the teeming square, another protester named Mortada, 21, watched the fanfare from a distance. “We want true electoral reforms. We want real change,” he said. “It’s not one man, it’s the whole system that needs to resign.”

Both Taif and Mortada declined to give their full names, fearing retaliation.

Protesters in the teeming square sang Iraq’s national anthem. One man held up a sign: “I cry blood for our martyrs.”

Nearly 400 people have been killed in the bloody crackdown on protests since Oct. 1, most of them young demonstrators who were shot or hit by exploding tear gas canisters fired by security forces.

In a statement, Abdul-Mahdi said he “listened with great concern” to Grand Ayatollah Ali al-Sistani’s sermon and made his decision in response to the cleric’s remarks.

“I will submit to parliament an official memorandum resigning from the current prime ministry so that the parliament can review its choices,” he said. Abdul-Mahdi was appointed Iraq’s fifth prime minister since 2003 as a consensus candidate following months of political wrangling between rival political blocs.

If accepted when put to vote, Abdul-Mahdi’s resignation would signal a return to square one in those slow-moving negotiations, Iraqi officials and experts said.

He would be the second prime minister in an Arab country to be forced out by mass protests recently.

Abdul-Mahdi’s rise to power was the product of a provisional alliance between parliament’s two main blocs — Sairoon, led by cleric Moqtada al-Sadr, and Fatah.