The volunteers cheered al-Sadr in the marble-floored courtyard of the Imam Ali mosque on Monday in an impressive show of force.
Al-Sadr is holed up inside one of Shia Islam's most sacred shrines before an expected American-led offensive.
Travelling to Najaf from across Iraq, the volunteers are swelling the ranks of his supporters and could provide another reason for US troops to think twice before storming the shrine.
"These people are a deterrent to the Americans because they are civilians. They are here so that the Americans won't attack the Imam Ali shrine," said Shaikh Ahmad al-Shaibani, a senior al-Sadr aide.
Any serious damage to the revered site would enrage millions of Shia around the world. However, US commanders have said they do not intend to attack the shrine itself.
The volunteers said they had no serious military training. But they seem ready to fire an AK-47 rifle or rocket-propelled grenade and use any means to try to block an advance by US tanks positioned in neighbourhoods near the shrine.
"I will lie on the ground in front of the tanks, or I will kill the Americans to defend al-Sadr and Najaf," said Fadil Hamid, 30.
"I will lie on the ground in front of tanks, or I will kill the Americans to defend al-Sadr and Najaf"
Fadil Hamid, volunteer
Last week, thousands of Iraqis staged pro-al-Sadr protests in several cities and called for the downfall of interim Prime Minister Iyad Allawi.
Several of the volunteers referred to Allawi as Saddam Hussein the second, referring to the toppled former president accused of killing thousands of Shia.
"Allawi you coward, you agent of the Americans," the crowd yelled. "Allawi we don't need you."