UR, Iraq (AP) -- With the worst of the fighting over, representatives from some of Iraq's often-quarrelsome factions met Tuesday in the biblical birthplace of the prophet Abraham for a U.S.-sponsored forum to begin shaping the country's postwar government.

Thousands of Shiite Muslims whose representatives were boycotting the meeting demonstrated in nearby Nasiriyah against the gathering.

The meeting took place at Tallil airbase, close to the 4,000-year-old ziggurat at Ur, a terraced-pyramid temple of the ancient Assyrians and Babylonians. The participants included Kurds, Sunni and Shiite Muslims from inside the country as well as others who have spent many years in exile. U.S. officials issued invitations to the groups, but each picked their own representatives.

Many Iraqis said they would boycott the meeting and opposed U.S. plans to install retired Gen. Jay Garner as head of an interim administration.

''Iraq needs an Iraqi interim government. Anything other than this tramples the rights of the Iraqi people and will be a return to the era of colonization,'' said Abdul Aziz Hakim, a leader of the largest Iraqi Shiite group, the Supreme Council for Islamic Revolution in Iraq.

U.S. officials hope more Iraqis join the process over time.

''It's critical that the world understand that this is only the fledgling first meeting of what will hopefully be a much larger series of meetings across Iraq,'' said Jim Wilkinson, spokesman at U.S. Central Command.

A national conference is planned ultimately to select the interim administration, perhaps within weeks, said a senior U.S. official, speaking on condition of anonymity.

The U.S.-led interim administration could begin handing power back to Iraqi officials within three to six months, but forming a government will take longer, said Maj. Gen. Tim Cross, the top British member of Garner's team.

''Will we get a complete government in place in that time? I doubt it,'' Cross said. ''One has to go through the process of building from the bottom up, allowing the leadership to establish itself, and then the election process to go through and so forth. That full electoral process may well take longer.''

Garner's Office of Reconstruction and Humanitarian Assistance is charged with coordinating humanitarian assistance, rebuilding infrastructure shattered by years of war and U.N. sanctions, and gradually handing back power to Iraqis leading a democratically elected government.

Tuesday's meeting is the first step toward that goal after the ouster of Saddam.

About 100 Iraqis were expected at Tuesday's meeting, half from inside Iraq, half exiles. The moderator was Zalmay Khalilzad, the White House envoy to Iraq. Garner was also expected, along with representatives from Britain, Australia and Poland -- countries that contributed forces to the coalition.

Wilkinson stressed that the meeting was an ''unscripted, free-flowing forum of ideas'' designed to get Iraqis talking about what they want for the future.

There are already tensions between the United States and some of the Iraqi factions.

Kurdish groups appear unwilling to compromise on their demand to expand the border of their autonomous area to include the oil-rich city of Kirkuk and the Kurdish parts of Mosul.

That could pose a problem for the United States, because Turkey worries that Kurdish control of Kirkuk could lead one day to aspirations for independence, which could also encourage separatist Kurds in Turkey.

Iraqi opposition leaders, meanwhile, fear the U.S. administration is using the meeting to try to force Ahmed Chalabi, head of the London-based umbrella Iraqi National Congress, on them as a leader of a new Iraqi administration.

Chalabi was the first top Iraqi opposition leader to be airlifted by the U.S. military into southern Iraq as the fighting wound down. U.S. officials said he was brought in because he offered forces to the coalition.

Neither Chalabi nor many other leaders of anti-Saddam groups are expected to attend: lower-level delegates are expected.

Thousands of Iraqi Shiites marched Tuesday through the streets of Nasiriyah to protest the U.S.-sponsored conference held outside the city. Organizers said the meeting did not represent their interests. Marchers chanted ''No to America and no to Saddam!''

The protesters held banners written in English and Arabic saying the ''Hawza,'' or the Shiite religious seminary in Najaf, represents them.

Even some of those attending the meeting said they did not want Garner leading the interim administration.

''We will press for any Iraqi civilian administration regardless of what the American say. An administration by Garner is not acceptable,'' said Mowaffak al-Rubaie, an Iraqi physician and opposition activist.