BAGHDAD, Iraq, Oct. 13 - Sunni Arab leaders who have endorsed the latest draft of the new constitution strongly defended their show of support today, saying recent compromises on the document will give Sunni Arabs more incentive to take part in elections scheduled for December.

The leaders made their remarks a day after several prominent Sunni Arabs denounced the compromises, and right after a powerful hard-line Sunni group, the Muslim Scholars Association, urged Iraqis to vote down the constitution.

Across Iraq, people are expected to walk by the thousands to polling stations on Saturday to vote on the constitution; support of the charter is considered a crucial step in the transition of this country's government to a fully realized democracy, although one steeped in Islamic values. The American military and Iraqi security forces prepared today to provide protection at polling centers, while the Ministry of the Interior announced harsh travel restrictions - the start of a 10 p.m. curfew, the closing of international land borders and Baghdad airport beginning Friday and a ban on almost all vehicle traffic on Saturday.

Prisoners in the two largest American-run detention centers, Abu Ghraib and Camp Bucca, were allowed to vote today, but there was no immediate estimate on how many actually cast ballots.

The draft of the constitution will pass with a majority vote, unless two-thirds of voters in three provinces choose not to back the document.

Even with some Sunni Arab leaders calling for a "no" vote, it is very unlikely that the constitution will be defeated.

Backing by prominent Sunni Arab groups, particularly the Iraqi Islamic Party and the Sunni Endowment, will probably split the Sunni vote. Until this week, Sunni Arabs were threatening a boycott or a unified "no" vote on the constitution, primarily because of fears that it could lead to too much power being given to regions or provinces. But on Tuesday, the Iraqi Islamic Party announced it had reached a compromise with Shiite and Kurdish leaders to allow for the possibility of a new parliament making far-reaching amendments to the constitution.

"We were keen that Iraq should be stable," Ayad al-Samarraie, a senior official in the Iraqi Islamic Party, said at a news conference inside the heavily fortified Green Zone this afternoon, as he sat next to several of the country's top Shiite and Kurdish leaders. "We didn't want this country to be in a state of turmoil because of constitutional dispute."

The most important compromise, approved by the National Assembly on Wednesday, allows for the new parliament to propose amendments to the constitution within the first four months of the parliament's formation. The amendment would then be put up for a national referendum if a majority of the parliament supports it. This new clause gives Sunni Arabs huge incentive to take part in mid-December elections for the new parliament, if only to try and get enough parliamentary seats to change the constitution, Mr. Samarraie said.

"They will take part in the National Assembly," he said of the Sunni Arabs.

The Shiite and Kurdish leaders sitting beside him urged voters to support the constitution, and one prominent Shiite politician, Hussain al-Shahristani, a nuclear physicist imprisoned by Saddam Hussein, said that the top Shiite ayatollahs in the holy city of Najaf were calling on Iraqis to vote on Saturday.

"They say that given the circumstances, this document is the best we can do to unify the political parties in Iraq right now," he said.

At the same time, in western Baghdad, dozens of Sunni Arab leaders filed into the Mother of All Battles Mosque to demand that voters do exactly the opposite on Saturday. "We call on the people to boycott the referendum or say no to the constitution," said Abdul-Salaam al-Kubaisi, an official with the Muslim Scholars Association, which says it represents 3,000 mosques across Iraq. "We reject the entire political system because of the presence of the invaders."

Many Sunni Arab politicians fear that the constitution as is written provides for the possibility of oil-rich Shiite and Kurdish regions to split off into autonomous areas, leaving the Sunnis with little more than large swaths of impoverished desert land.

It is in those Sunni areas, considered strongholds of the insurgency, that the American military and Iraqi security forces are bracing for what could be a surge of violence over the next few days.