NAJAF, Iraq (Reuters) - With his militants and human shields holed up inside one of Shi'ite Islam's most sacred shrines, radical Iraqi cleric Moqtada al-Sadr is playing a shrewd waiting game before an expected American-led offensive.

Sadr's militiamen were inside the Imam Ali shrine and positioned along alleyways and on rooftops with a seemingly endless supply of AK-47 rifles and rocket-propelled grenades intermittently fired at U.S. troops in a nearby cemetery.

But it was about 2,000 impassioned Iraqi civilian "volunteers" cheering Sadr in the marble-floored courtyard of the mosque who made the biggest show of force Monday.

Traveling to Najaf from across Iraq, they are swelling the ranks of Sadr's supporters and could be another reason why U.S. troops may 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 Sheikh Ahmed Shaibani, a senior Mehdi Army commander and top aide to Sadr.

The longer the Americans wait to launch any offensive, the more time Sadr has to gain new supporters and entrench them inside the sprawling mosque.

Any serious damage to the shrine would enrage millions of Shi'ites around the world, including those who make up about 60 percent of Iraq's population.

The volunteers said they had no serious military training. But they seem ready to pick up an AK-47 rifle or use any means to try to block an advance by U.S. tanks positioned in neighborhoods near the shrine.

WILLING TO DIE

"I will lie on the ground in front of the tanks, or I will kill the Americans to defend Sadr and Najaf," said Fadil Hamed, 30, standing among a group of men who said they walked to Najaf from the southern city of Basra.  

Last week, thousands of Iraqis staged pro-Sadr protests in several cities and called for the downfall of interim Prime Minister Iyad Allawi.

Some marched to Najaf and are in the shrine with many of Sadr's Mehdi Army militiamen, who are posing the biggest challenge to Allawi since the U.S.-led occupiers handed power to Iraqis in late June.

Beside a makeshift emergency clinic inside the mosque, a woman in a black veil comforted a man who lay on the marble floor with wounds to his right leg and arm.

During fighting Sunday, a shell landed outside the mosque, famous for its bright gold dome, and wounded him and another person and killed a third.

But that did not dampen the enthusiasm of Sadr's volunteers who spent hours in the blistering sun chanting his name as his men fired mortars.

"We will not leave the shrine until the Americans get out of Najaf. We will kill," said Ugil Abdel Hussein, 32, a member of the Turkmen minority from the northern city of Kirkuk.

Sadr is also looking to Iraqi tribes to advance his cause. Hundreds of tribesmen met inside the shrine and vowed their support.

One of them called on all of Iraq's tribes to hold a national conference Wednesday to end the Najaf crisis.

"All Iraqi tribesmen are invited to try and end this bloodbath in Najaf," said Sheikh Kassim Khafaji, addressing more than 100 tribesmen.

As gunfire crackled around the shrine, two large water delivery trucks suggested Sadr and his followers were digging in for the long haul.

Teenage boys with grenades strapped to their backs take cover in small alleyways beside shuttered shops.

The consequences of a raid by Iraqi security forces or Americans that seriously damages the mosque or harms Sadr are clear in the tense streets of Najaf.

"The Americans are applying military pressure on us to try and make us weak so they can get concessions. It will not happen. We are ready to fight and we are very patient," said Shaibani.