Powerful Shi'ite cleric Muqtada al-Sadr told tens of thousands of supporters that Iraqi Prime Minister Haidar al-Abadi must root out corruption and carry out promised reforms or risk losing power.
A crowd of as many as 100,000 people flocked to Baghdad's Tahrir Square to join a protest Sadr called against corruption on February 26, far more than the few hundred people who have attended previous weekly protests.
Abadi last summer promised political and economic reforms after mass street protests, but quickly ran into legal challenges and entrenched resistance to change.
This month he vowed to appoint technocrats to replace ministers appointed on the basis of political affiliations, but that promise also remains unfulfilled.
"Today the [position of the] prime minister is at stake, especially after the people have revolted," Sadr said at the rally.
Chanting anti-American slogans to energize the huge crowd, Sadr accused Abadi of failing to capitalize on the support offered by Iraq's highest Shi'ite religious authority, Grand Ayatollah Ali al-Sistani, who endorsed Abadi's proposed reforms last year but appears to have grown frustrated with their lack of progress.