Iraqi security forces raided Baghdad's main protest site on Saturday and tried to eject protesters in southern cities, firing tear gas and bullets, killing four people and wounding dozens more, police and medical sources said.
His followers had presented themselves as protectors of the protesters, whether that was the case or not, so their withdrawal implicitly opened the way for opponents of protesters, including Iraqi government forces, to move.
A statement by the Baghdad Operations Command (BOC) said that the security forces reopened al-Ahrar Bridge and the squares of Tayran and Cordoba and nearby highway named Mohammed al-Qassim, reports Xinhua news agency.
Three more died and 14 more were wounded as security reclaimed a bridge in the southern city of Nasiriya.
The actions appeared to be an attempt to fully clear out anti-government sit-ins and end months of popular demonstrations that have called for the removal of Iraq's entire ruling elite.
The young demonstrator was shot dead by unidentified assailants after he left the main protest camp in Basra, a security source said.
Al Jazeera's Imran Khan, reporting from the protest, said the turnout was a "real show of strength".
More than 470 people have died in protest-related violence since October, a lot of them demonstrators, and violence has spiked this week.
"The demonstrations have become stronger now because of what happened", said Zaidoun, 26, a protest organiser in Baghdad.
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Security forces used the tactics at Khilani Square, near Baghdad's Tahrir Square, at about noon, medics said.
"(Al Sadr's statement) gave the green light for the government to suppress the demonstrations", said Husanien Ali, a 35-year-old protester.
Baghdad has been gripped by kidnappings and shoot-outs which protesters say are meant to intimidate them.
Tens of thousands of Iraqis, mainly al-Sadr's followers, gathered to reject the presence of USA troops in Iraq.
Iraq also has been roiled by U.S. -Iran tensions that reached fever pitch when an American drone strike this month killed top Iranian Gen. Qassem Soleimani outside Baghdad's airport. "This is considered a betrayal by the al-Sadr bloc".
Bolstered by his own protest on Friday, Sadr announced he was dropping support for the youth-dominated reform campaign rocking the country since October.
At least 22 demonstrators were reported wounded by Iraqi security forces in the first hours of Sunday's street rallies.
The killing, to which Iran responded with ballistic missile attacks on two Iraqi bases hosting United States troops, has revived tensions in Iraqi politics and delayed the formation of a new government. "He was merely expressing his disappointment in those in Tahrir Square who have been criticizing him and his motives".
The missiles hit Baghdad's heavily-fortified Green Zone, AFP said, citing security sources.