Carrying assault rifles, homemade rocket launchers and missiles, row after row of men in combat fatigues marched through the streets of Baghdad on Saturday, signaling the resurgence of one of Iraq’s most feared Shiite militias.

Tens of thousands of fighters loyal to the firebrand cleric Moqtada al-Sadr packed the streets of Sadr City, the Shiite neighborhood in the capital named after his father. Other rallies were held in cities to the south.

The marches effectively signaled a return to arms for the Mahdi Army, the powerful militia led by Sadr that once spearheaded an armed campaign against U.S. troops in Iraq and is now regrouping in the face of an al-Qaeda-inspired insurgency.

While the fighters marched in Baghdad, their enemies from the Islamic State of Iraq and Syria (ISIS) seized a key border crossing with Syria. Its capture gave ISIS control of most of the Iraq-Syria border, erasing the line drawn by colonial powers to demarcate the two nations in the wake of World War I.

“The Mahdi Army is rising again,” said Zaboun Ali, a 52-year-old dressed in combat gear. A few yards away, lines of black-shirted men carrying rocket-propelled grenades jogged past.

“This is the army that will protect the country,” he said, gesturing toward them.

Their show of strength as the Iraqi state increasingly relies on irregular forces to repel Sunni insurgents heightened fears of sectarian conflict.

The militiamen had answered a call from Sadr to form “peace brigades” to protect Iraq’s shrines and holy sites as ISIS pushes forward in the country’s north. But the Mahdi Army’s display Saturday was a clear signal of aggression.

The Mahdi Army’s activities have been frozen since 2008, after a period of violent sectarian conflict during which its members ran death squads targeting Sunnis and battled U.S. soldiers.

In a reminder of the challenge they mounted against the U.S. presence, the volunteer fighters displayed a range of improvised explosive devices Saturday. Those included explosively formed projectile, or EFP, bombs, which are able to punch through the armor of a tank and proved deadly against U.S. forces in their last years in Iraq.

“We have two messages,” said Hakim al-Zamili, a member of parliament from Sadr’s political party who wore military fatigues as he watched the parade. “One is a message of peace, that we will defend Iraq and its shrines. The second is a message of terror to ISIS and al-Qaeda.”

Announcing the “peace brigades” a day after the fall of the northern city of Mosul, Sadr stressed that he was not willing to fight a “dirty militia war.” His call to arms has been backed up by a religious decree by Iraq’s leading Shiite cleric, Grand Ayatollah Ali Sistani, though Sistani has urged volunteers to join the security forces rather than militias.

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Sadr has said he is willing for his men to work with Iraq’s security forces “temporarily,” as the state attempts to fend off the insurgency.

The United States is increasing its assistance to the Iraqi military, sending 300 advisers despite the increasingly blurred lines between Shiite militias and the country’s security forces.

Although U.S. interests are overlapping with the Shiite militias’ as both attempt to push back the ISIS advance, anti-American sentiments ran high Saturday.

“No to America! No to Israel!” the militiamen chanted.

“If America is going to send troops back to Iraq, we are a time bomb waiting for them,” said Adel Jabbar al-Bawi, a 41-year-old merchant, sitting on the back of a truck after taking part in the march. “We will eat them alive.”

Others said they would welcome assistance in the form of airstrikes, as ISIS militants pressed their offensive against crumbling Iraqi government troops.

The al-Qaeda offshoot on Saturday captured the crossing of Qaim on the border with Syria, dealing another setback to the shrinking authority of Prime Minister Nouri al-Maliki’s government. About 30 Iraqi troops were killed as they attempted to repel the attack 200 miles west of Baghdad, the Associated Press reported, citing security officials.

The extremists control areas around the Iraq-Syria border, across which they have freely moved fighters and equipment for months. But control of the official border crossing will facilitate deployments of equipment and also advance their goal of eroding the border between the two nations in pursuit of an Islamic state.

At the rally Saturday, Zamili asserted that with Sadr’s followers remobilized, there is no chance of that.

“The Mahdi Army has huge numbers of fighters,” he said. “They are well trained and equipped, and experienced from their victories against the Americans.”

Referring to ISIS and al-Qaeda, he warned, “We will kill them, horrify them, to defend our country.”

Liz Sly contributed to this report.