BOB ABERNETHY, anchor: In Iraq this week, dramatic scenes of religious penance and fervor as Shiite Muslims displayed their power and prompted hard questions for the U.S.

The U.S. says it wants democracy in Iraq. But what if Muslim clerics -- and a majority of Iraqis -- were to prefer a strict Islamist theocracy? Secretary of Defense Rumsfeld warned this week, "That isn't going to happen."

Lucky Severson has our report on religion and politics in postwar Iraq.

LUCKY SEVERSON: It was a spectacle not seen in 25 years -- more than a million Shiite Muslims on a pilgrimage to the holy city of Karbala and the shrine honoring the prophet Muhammad's grandson, Hussein bin Ali. Some walked barefoot 60 miles to get here and then they performed the ritual of flagellation and self-mutilation as part of their religious observance. During the oppressive regime of Saddam Hussein, himself a Sunni Muslim, such a pilgrimage was forbidden and dangerous for those who tried it.

Judith Yaphe is with the National Defense University in Washington.

Professor JUDITH YAPHE (National Defense University): Anyone who tried to do the march to Karbala or participate in these kinds of ceremonies would have been arrested, interrogated, tortured, perhaps might have disappeared.

SEVERSON: But the good news for Iraq's Shiite Muslims may turn out to be bad news for the Bush administration. The Pentagon had counted on Iraq remaining a secular state, and on the support of the country's Shiites because they were often the target of Saddam's brutality.

So it was not a pleasant surprise when thousands of Shiites chanted anti-American slogans, some calling for an Islamic state.

Professor SHAUL BAKHASH (George Mason University): I think the speed with which the clergy were able to organize these huge demonstrations was surprising.

SEVERSON: Shaul Bakhash is a history professor at George Mason University in Virginia.

Prof. BAKHASH: The assumption was after two decades of Saddam Hussein's oppression, even their organization and network had been undermined and weakened.

SEVERSON: And it's not only the Shiites, who make up 60 percent of Iraq's population of 23 million. The Sunni Muslims have shared protests with Shiites and organized their own anti-American demonstrations.

Prof. YAPHE: There has also been a call, I understand, in some of the Sunni mosques, also saying, "We want to live in an Islamic state and the occupiers must leave."

SEVERSON: Things began to go wrong almost immediately when a cleric favored by the U.S. was stabbed to death at a shrine in Najaf a few weeks ago. And now there is widespread skepticism that the Pentagon's original choice to lead the interim government, Ahmed Chalabi, will ever be accepted, because he hasn't lived in Iraq in 45 years, and because he was hand-picked by the Pentagon.

Prof. BAKHASH: The U.S. does have clout at the moment. They have a large military presence in Iraq. But again, the ability of those that don't want to see an American presence to be spoilers in these plans is quite considerable.