The offensive, led by Sunni Arab militants from the Islamic State of Iraq and the Levant group but involving other organisations as well, has overrun swathes of northern and central Iraq this month.
Iraq's flagging security forces, which were swept aside by the initial offensive but have since at least somewhat recovered, have already been joined by some Shiite fighters, and thousands more are ready to take part.
The parade, which was called for by Sadr, began with mud-smeared trucks mounted with tubes for launching rockets.
Armed guards watched over the parade, most with assault rifles but some carrying heavier weapons.
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Some units shouted "Mahdi", the name of the 12th revered Shiite imam, every four steps as a cadence as they marched.
One unit chanted the full name "Jaysh al-Mahdi", or "Mahdi Army" -- Sadr's officially inactive militia that battled American forces in past years.
Some of the fighters at the lead of units carried Iraqi flags, while others held signs with messages including "We sacrifice for you, oh Iraq," "No, no to terrorism," and "No, no to America".