KARBALA, Iraq (KRT) - A massive, weeklong Shiite Muslim pilgrimage to this holy city turned against the United States on Wednesday with fierce anti-American outpourings and a demand by a senior cleric that U.S. troops leave Iraq "as soon as possible."

It was the latest sign that once-downtrodden Shiite religious leaders are competing with U.S. forces to win control of post-war Iraq.

Upheaval among Shiite Muslims, who make up 60 percent of Iraq's population, confronts Washington with a major political challenge of preventing efforts to form a theocracy in Iraq and limiting the influence of neighboring Iran, which is tightly controlled by fundamentalist Shiite clerics.

Amid reports that Iranian agents are filtering across the border and that a senior exiled ayatollah will soon arrive from Iran, the White House warned Tehran against meddling.

Karbala, a city sacred to Shiites, was the center of joyous celebrations over the past week as hundreds of thousands of Muslim pilgrims arrived on foot to a gold-domed mosque to pay tribute to Ali Hussein, the prophet Mohammed's grandson, who was murdered more than 1,300 years ago. They shouted with passion - ecstatic to be able to commemorate a major part of their history that was banned for decades by Saddam Hussein.