In Iraq, where followers of a radical Shia cleric have been battling U.S. forces for days, the country's most revered Shia religious leader has been not only silent, but absent. Grand Ayatollah Ali al-Sistani has left Iraq for medical treatment in London.

In the middle of the Baghdad neighborhood of Kadhimiya is a shrine holy to Shia Muslims. Its golden dome and minarets glisten in the summer sun, as the call to prayer rings out from many smaller mosques nearby.

Kadhimiya is an overwhelmingly Shia neighborhood, and most of its residents follow the religious teachings of Grand Ayatollah Ali al-Sistani, Iraq's most senior Shia cleric.

In a tiny shop selling gold jewelry, Ammar Adnan al-Hashemi explains the intensity of his devotion.

"If you ask me who my leader is, I will tell you Ali al-Sistani is my leader. He can order us to do anything," he said. "If he said throw yourself in the river, I would throw myself in the river."

Ali al-Sistani is one of the most senior and most respected Shia clerics in the world, not just in Iraq. He is 73 years old and was born in Iran, but came to Iraq as a young man to study in the holy city of Najaf, where he still lives.

He is considered a moderate, and keenly intelligent leader who commands the respect even of secular Iraqis, such as political science professor Nabeal Younis of Baghdad University. "The man is wise? We need such a man in Iraq, especially in this period," he said.

His tremendous popularity among Shia Muslims, who make up 60 percent of Iraq's population, makes Ayatollah al-Sistani the most influential cleric in Iraq. But despite that, he has refused to take any overt political role and prefers to stay out of politics, at least publicly. Ayatollah al-Sistani believes religious leaders should deal with matters of religion, and leave the running of the country to politicians.

Not all of Iraq's Shia clerics feel the same way. Some are known to advocate an Iranian-style government, where religious leaders are the ultimate authority. But the moderate and liberal clerics, including Ayad Jamal al-Din, support the separation between religion and the state.

"We consider Sayed Ali al-Sistani to be a supreme religious scholar and leader of religious people. But there are other leaders for politics," he said.

Despite the ayatollah's desire to stay out of the political arena, he has been deeply involved in many aspects of Iraq's transition. Members of the now-defunct Governing Council were known to keep him informed about their debates and decisions. American officials have understood that their decisions have had to be largely acceptable to him, because his rejection of the transitional process could have spelled disaster if it sparked an uprising by his followers.

For example, the United States originally wanted to have Iraq's new constitution written first, and then hold elections. But Mr. al-Sistani insisted that the people who write Iraq's constitution must themselves be elected, and his opposition to the U.S. plan forced them to change it.

The political scientist, Mr. Younis, believes Ayatollah al-Sistani played an important role in negotiating the timetable. "I think the Americans were not happy with the attitudes of Sistani, or any other political or religious leader in Iraq, as far as it is against the occupation," he said. "But they had to listen to them."

But Mr. al-Sistani has been very careful about how he has dealt with the Americans. He has generally urged his followers to remain peaceful, unlike the radical Shia cleric Moqtada al-Sadr, who has called twice for open rebellion and has vowed to fight to the death.

In fact, it was Ayatollah al-Sistani who helped broker the cease-fire that ended al-Sadr's last uprising. That is why many people are curious about the timing of his unexpected trip to London for medical care.

The famously reclusive cleric, who almost never leaves his offices in Najaf, suddenly flew to London last Friday to receive treatment for a heart condition. It was the same day that all-out battles began between Mr. al-Sadr's militia and multi-national troops in Najaf.

Since then, the fighting has intensified, especially in a vast cemetery near the Shrine of Imam Ali, one of the holiest sites in Shia Islam.

Professor Younis believes the cleric is remaining intentionally silent, although he is not sure why.

"If he wants to play such a role, he can deliver his speech from London to Baghdad. But it seems to me that the man chooses to be silent in this period," he said.

Many Iraqis, including Kadhimiya resident Ayad Sayeed Kadhim, believe he left at the request of the U.S. military, to prepare for an all-out assault on the Sadrist militia.

"I think this is a plan of the new Iraqi government and the Americans, to clear the way for military action. If Sayed Ali al-Sistani was still in Najaf, this government could not do anything without his consent," he said.

Iraqi officials, however, have strongly denied that charge.

There is also disagreement about the Mr. al-Sistani's health, and about what might happen if he were to die. Some people believe there would be a power struggle, but others say there is a fairly short list of senior clerics with enough experience to take his place, and they predict a smoother transition.

What is not clear is whether his successor would have the same attitude toward the separation of religion and politics as the esteemed Grand Ayatollah.