Samarra, a Sunni-majority city 80 miles (125 kilometres) north of the capital and a former hotbed of Iraq's insurgency, houses a major Shiite mausoleum blown up by suspected Al-Qaeda forces in 2006.
Sadr, head of the Mahdi army, last month urged the faithful to restore a traditional pilgrimage halted since the 2003 US-led invasion and to pray at Samarra's golden-domed shrine to commemorate the death of the 11th imam, Hassan al-Askari, at his mausoleum.
The commemoration falls this year on March 6.
Iraqi television stations reported the gathering swelled to more than a million.
"About 800,000 pilgrims arrived in the city," said Samarra governor Hamad Homoud al-Shagti well before midday as crowds continued to pour in.
"The security situation is good and there have not been any security violations so far," he told reporters. "Iraqi forces are deployed along the roads."
"Iraqi and US air forces are hovering over the area" in helicopters, Shagti said.
Police Major General Rasheed Flayeh said cars had been banned from the city to prevent bombings.
No serious incidents had been reported by mid-afternoon.
Tens of thousands of people died in violence sparked by the destruction three years ago by alleged al-Qaeda Sunni extremists of the dome of the revered Al-Askari mosque, built in 944. The golden dome was added in 1905.
Although security has improved in Iraq recently, dozens of Shiites were killed last month in bomb blasts as they headed into the holy city of Karbala, south of Baghdad, for a major religious ceremony.
AFP - 6 march 2006