Sayed Sadiq al-Hashemi, the director of the Iraqi Center for Studies, said more than 2.5 million took part in the demonstrations on Friday.
Since the early hours on Friday, huge crowds of men, women and children of all ages converged on the Jadriyah neighborhood near Baghdad University.
The protesters were seen carrying banners and chanting slogans calling for the expulsion of US forces.
"Get out, get out, occupier!" some shouted, while others chanted, "Yes to sovereignty!"
Iraq's al-Ahd news network reported that Iraqis from all of the country's provinces had gathered in the city.
On January 5, the Iraqi parliament voted overwhelmingly in favor of a resolution calling for the expulsion of all foreign forces after the US assassination of Iran's General Qassem Soleimani and his Iraqi trenchmate Abu Mahdi al-Muhandis.
The massive rally came after influential cleric Muqtada al-Sadr called on Iraqis to stage "a million-strong, peaceful, unified demonstration to condemn the American presence and its violations".
Sadr issued a statement on Friday calling for US bases to be shut down and Iraqi airspace closed to US warplanes and surveillance aircraft.
He warned that US presence in the country will be dealt with as an occupying force if Washington does not agree with Iraqi demands to withdraw for the country.
In a message delivered through a representative at Friday prayer in the holy city of Karbala, top cleric Ayatollah Ali al-Sistani also urged Iraqi political groups to do what is needed to the safeguard the country's sovereignty.
He called on Iraqi groups to stand united, far from any foreign influence in countering the dangers which threaten the country.
On Thursday ahead of the planned rallies, Sadr called on Iraqis to mobilize and defend the country's independence and sovereignty.
"Oh women, men and youth of the country, the time is now upon us to defend the country, its sovereignty and dependence," Sadr said in a tweet, Press TV reported.
"Spread the word of an independent future Iraq that will be ruled by the righteous; an Iraq which will not know of corruption nor aggression" he added, calling on Iraqis to expel the "tyrants".
Various Iraqi resistance groups affiliated with the country's Popular Mobilization Units (PMU) have also backed the anti-American rally.