The fighters signalled their readiness to take on the Sunni militants of Isis as they took to the streets with weapons including a new three-metre missile named after Sadr.
Sadr called for a military show of strength across the country.
"They are the brigades of peace. They are ready to sacrifice their souls and blood for the sake of defending Iraq and its generous people," one man commented as the troops marched by.
Officials admitted Sunni militants, led by jihadist group Isis, seized a strategically important border crossing to Syria, near the town of Qaim.
It signalled another blow to Iraqi Prime Minister Nouri al-Maliki's government, which has struggled to push back against Islamic extremists who have seized large swathes of the country's north.
But the display of heavy weapons by the Shia fighters indicated that forces beyond Baghdad's control may be pushing the conflict toward a sectarian showdown. Police and army officials said Isis, and its supporters, seized Qaim and its crossing, about 200 miles west of Baghdad, after killing 30 Iraqi troops.
Chief military spokesman Lt Gen Qassim al-Moussawi told a news conference in Baghdad that troops aided by local tribesmen were seeking to clear the city of "terrorists". He said fighting was continuing for a fifth day over Iraq's largest refinery in Beiji, north of Baghdad, with the army there repelling three waves of attacks by the Sunni militants on Friday night.