A high-profile Shiite official who led efforts to purge Iraq’s national government of loyalists to Saddam Hussein was shot and killed Thursday night, security officials said, becoming the latest victim in an escalating series of attacks on the country’s political and security leaders.

Ali al-Lami, head of Iraq’s Justice and Accountability Commission, was riding in a car about 8 p.m. in eastern Baghdad when gunmen approached and shot him in the head, officials said.

Lami’s efforts to remove members of the Baath Party from the government made him one of the most controversial figures of the post-Hussein era, and his death will probably return attention to the perilous state of security and politics in Iraq more than eight years after the U.S.-led invasion.

The killing occurred on a day that included thousands of young men marching through Baghdad’s Sadr City in a show of force apparently intended to prove that they could restart the Shiite insurgency if U.S. troops do not leave the country by the end of the year.

The parade by Moqtada al-Sadr’s Mahdi Army lasted for hours, as the participants — displaying new uniforms resembling the Iraqi flag — marched past tens of thousands of well-wishers supporting Sadr’s call for U.S. forces to abide by their scheduled Dec. 31 departure.

The protesters burned and kicked replicas of American and Israeli flags and carried signs reading “No, No America” and “No, No Israel.”

Although Sadr’s militia was unarmed and the march remained peaceful, bombings and mortar attacks continued across the country Thursday, including at the heavily fortified Green Zone and Baghdad International Airport. No injuries were reported at those sites, but at least nine Iraqi police or military officials were killed elsewhere in addition to Lami.

As the right-hand man of the even more controversial Ahmed Chalabi, a Shiite opponent-in-exile of Hussein and onetime U.S. ally, Lami was put in charge of the de-Baathification committee created by U.S. administrator L. Paul Bremer in 2003. He oversaw the dismissal of thousands of government workers and military officials from their jobs because of their former membership of Hussein’s Baath Party.

Lami was loathed by Iraq’s Sunnis for his perceived persecution of Baathists and for his alleged ties to Iran, and the Baathist-led Sunni insurgency would have had many reasons to kill him. Observers noted, however, that at a time of deep splits within the Shiite community, it is also possible that he was a victim of internecine Shiite rivalries.

But Entifadh Qanbar, a spokesman for Chalabi, immediately blamed Baathists loyal to the Hussein government, saying that Lami appeared to have been “well-monitored and traced” by his attackers.

“We lost a brother, and we lost a very important role model who played a huge role in uprooting the Baath Party from society,” Qanbar said. “But we support freedom in Iraq, support freedom everywhere, and that is how we work, and killing each member is not going to stop us from doing what we need to do.”

The attacks on Iraq’s fragile government is generating growing uncertainty about what will happen this summer as U.S. and Iraqi officials struggle to avoid more bloodshed.

In the coming weeks, the Obama administration and Iraqi Prime Minister Nouri al-Maliki must decide whether any of the 46,000 U.S. troops in the country will stay beyond the end of the year to help with training and security.

In 2008, after helping feed the insurgency that followed the U.S-led invasion, Sadr told his forces to lay down their arms. But the firebrand Shiite cleric, who recently returned to Iraq after four years of self-imposed exile in Iran, has suggested that the order could be lifted if U.S. troops remain.

According to the Associated Press, as many as 70,000 people took part in or watched Sadr’s parade Thursday, the Mahdi Army’s first organized public event in Baghdad in several years.

“We just hope America is going to withdraw,” said Hayda Qasim, 42, a carpenter from Sadr City who was watching the parade. “We are not thirsty for more blood. We just don’t need any more interference in our affairs. . . . It’s been eight years. It’s over.”

Maj. Gen. Jeffrey S. Buchanan, chief spokesman for the U.S military in Iraq, said Thursday that every party in a democracy has a right to be heard but criticized the parade as an “attempt to influence a debate by coercion.”

“Regardless of one’s political beliefs, nobody finds comfort in an open threat to bring back an illegal militia,” he said.

Sadr did not appear or speak at the parade, although his spokesman said he had traveled from his home in southern Iraq to Sadr City for the event. According to the spokesman, Sadr decided not to get out of his car and address the crowd for fear of triggering a stampede.

The cleric’s influence was palpable at the gathering, however, with militia members and spectators emphasizing that he will determine their relationship with U.S. forces and Iraqi politicians.

“This is a peaceful demonstration against American occupiers. Sadr asked us to remain peaceful,” said Salah Emarah, 35, who traveled from his home in the southern port city of Basra to march in the parade. “At the end of the year, according to Iraq, the occupiers will leave. If they don’t, we will wait for orders.”

Correspondent Liz Sly in Beirut and special correspondents Aziz Alwan and Asaad Majeed in Baghdad and Saad Sarhan in Najaf, Iraq, contributed to this report.