BAGHDAD — They came from the slums of this city’s underclass, the alleyways and the simple halls of the seminary in the Shi’ite holy city of Najaf, and the outer reaches of the rural south.

They waved Iraqi flags and demanded change. The crowd packed Baghdad’s Tahrir Square on Friday morning, chanting by the tens of thousands against corruption and for decisive reforms in how politics is conducted here, as they waited for their man to appear.

“No, no to thieves! Yes, yes to reforms!’’

Then Moqtada al-Sadr, the cleric and political provocateur whose command of the Iraqi Shi’ite street is unmatched, stepped up to the rail of a makeshift stage on the rooftop of a former school for girls and appealed to the people’s grievances in terms at once revolutionary and patriotic.

“After today, the prime minister has to act!’’ he said. Referring to the barricaded heart of the central government, he said, “Today we are at the door of the Green Zone, and tomorrow the people will be inside!’’

The time is ripe for demagogues again in Iraq, where the public is seething with anger over corruption, a grinding war, and a collapse in oil prices that has shaken the economy. With an ineffective political class unable to rise above internal scheming, Iraq is struggling to face its most pressing concerns, the primary one being winning the war with the Islamic State and reuniting the country.

Sadr and his fearsome militia were once a primary enemy of the United States, and he played many roles in shaping Iraq after the US invasion: populist cleric, Iranian proxy, Iraqi patriot, political kingmaker. In seizing a chance Friday to return to the political spotlight, he positioned himself as an Iraqi nationalist in the face of Iran’s growing role and as an ally to a weak prime minister.

“Today I am among you to say to you, frankly and bravely, that the government has left its people struggling against death, fear, hunger, unemployment, occupation, a struggling economy, a security crisis, bad services and a big political crisis,’’ Sadr told the crowd.

Above all, it was a reminder of Sadr’s complexity, and the confused state of internal Shi’ite politics, that even as he was seeking to harness public rage against the political elites, he had actually called the street rally to support the reform policies of the country’s struggling prime minister, Haider al-Abadi.

Abadi’s proposal to tackle corruption and install technocrats in the country’s ministries has stalled over the opposition of powerful militia leaders and some pro-Iran politicians. For his part, Sadr has offered to have his ministers resign in protest to lend Abadi’s agenda some steam. Despite that, and the support of the most senior Shi’ite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, it remains unclear whether Abadi’s agenda will be able to win the help of any other political blocs.

Sadr was once at the very heart of things in Iraq, but in recent years had receded somewhat from the public eye.

When the United States invaded in 2003, Sadr was just shy of 30. But he drew on the political inheritance of his father, a pivotal and immensely popular Shi’ite cleric assassinated on Saddam Hussein’s orders in 1999, to emerge as a powerful voice for the Shi’ite underclass.

He forged a movement that melded martial, political and social elements. His militia, the Mahdi Army, once fought the Americans and the Iraqi state, and it was blamed for atrocities during the sectarian civil war of 2006 and 2007.

Nowadays his militiamen are largely under the control of the government, and his anti-Americanism, once a defining issue for him, is less ardent. Recently, through his contacts among Shi’ite militia leaders, he helped secure the release of three Americans kidnapped in Baghdad. At the rally there was some bashing of the United States, but it felt more perfunctory than strident.

Once an open client of Iran, Sadr has in recent years gone his own way, and is widely seen these days as an Iraq-first advocate of cross-sectarian unity. His militia, reconstituted after the extremists of the Islamic State captured Mosul in summer 2014, was renamed the Peace Brigades.

Today, as he seeks to redefine himself once again, Sadr, now 42, has positioned himself as backer of Abadi, who is seen as increasingly weak in the face of the growing influence of Iran. Tehran supports Abadi’s political rivals, who command militias.

“Abadi, as a person, is kindhearted,’’ said Saad Thamer, 37, a supporter of Sadr’s who attended the rally. “But he is very weak.’’

The militias have become exceedingly popular among the Shi’ite public, challenging al-Abadi’s authority, because they are seen as the protectors of the Shi’ite majority against the Sunni extremists of the Islamic State.

It has been a challenge to Sadr, too, who has lost of some of his support at the grass-roots level as young men flock to other militias seen as more powerful. His embrace of the Iraqi state has also sometimes worked against him by contradicting his image as a populist figure.

“From an antiestablishment young leader, he compromised his stance by working more with the Iraqi political establishment, which cost him a loss of some popularity among his followers,’’ said Maria Fantappie, the Iraq analyst for the International Crisis Group.

Iraq is a place where everyone has his enemies, and Sadr has his share. One of his chief critics is the former prime minister, Nouri al-Maliki, who once counted on Sadr’s support to secure a second term after national elections in 2010.

Sadr later broke from Maliki, and tried to oust him from the premiership. In a recent interview, Maliki said Sadr was seeking to exploit Abadi’s weakness to reestablish his own influence.

“He’s supporting Abadi, but in his own way,’’ Maliki said. “He wants to control Abadi.’’