"Stop destroying Syria as you destroyed our country," shouted protesters in Baghdad's Tahrir Square, in reference to the 2003 US-led invasion of Iraq.
"No to America, no to the bombardment of Syria," they chanted.
Protestors set light to US flags as they chanted against the military action by the US, France and Britain.
Crowds also took to the streets in the cities of Najaf and Basra, south of Baghdad.
Sadr -- whose militias battled US forces after the 2003 invasion -- joined the protest in Najaf, where he lives.
After withdrawing from Iraq in 2011 after a years-long occupation, US troops returned to the country in 2014 to help fight the Islamic State group.
American military support at the head of an international coalition helped Iraqi forces to roll back the jihadists and eventually declare victory over IS in December.
The Iraqi government warned Saturday that the Western air strikes on Syria were a "very dangerous" development that could fuel a jihadist resurgence in the region.
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