Iraq's Sunni Arab minority made a violent reentry into politics Friday, bombing offices of a political party that urged support for a new U.S.-backed constitution while posting insurgents and tribal fighters at some polling places to ensure that Sunni voters could vote safely Saturday against the proposed charter.

Approval of the charter in the referendum, which would open the way for Iraq to be remade as a loose federation under expanded religious authority, appeared almost certain despite strong Sunni opposition. Members of the country's Shiite Muslim majority and their Kurdish political allies overwhelmingly support the proposed constitution, which was drafted by leaders of their own political parties. At the same time, many Iraqis, confused and alienated by last-minute revisions, say they will vote neither for nor against a charter that almost no one has seen in its final form.

In part of a referendum-eve blitz on state-funded television to drum up turnout, President Jalal Talabani promised a true coalition government if Sunni Arabs turned away from their two-year-old insurgency and joined in the political process Saturday. "Ever after, Iraq will be unified and strong, an independent and prosperous nation in which all people enjoy freedom and equal rights," said Talabani, an ethnic Kurd.

Sunni Arabs, who account for an estimated 20 percent of Iraq's population, have largely boycotted politics since the fall of President Saddam Hussein and his Sunni-dominated Baath Party in April 2003. But the referendum has both galvanized and divided the disaffected minority.

Throughout the day Friday, bombs in Baghdad and western towns tore through offices of the leading Sunni political party, the Iraqi Islamic Party, which formally broke with most other Sunni groups Thursday to call for the draft's approval.

While the attacks were going on, branches of the party in the west announced they were splitting with the headquarters in Baghdad. In Fallujah, crowds gathered around an Iraqi Islamic Party office set ablaze by guerrillas from Abu Musab Zarqawi's al Qaeda in Iraq movement, chanting "No!" to the constitution.

At the main Sunni mosque in the largest western city, Ramadi, fistfights broke out between supporters and opponents of the draft charter.

Overnight, families who live near polling centers in the Sunni-dominated west packed up and fled after two days of bomb attacks on the voting sites by insurgents. "I fear what will happen Saturday," said Emad Ahmed, head of one of a dozen families who evacuated one neighborhood in Ramadi on Friday.

Al Qaeda in Iraq, led by Zarqawi, a Jordanian, and infused with foreign fighters, distributed leaflets in the west pledging punishment for all who voted Saturday. "We have warned; we shall not be blamed," one leaflet read.

But six insurgent groups led by Iraqis countered with a call for restraint by their foreign allies. In one of the first signs that some Iraqi insurgents were eschewing violence for politics, Muhammad's Army, the Mujaheddin Army and other organizations based in Ramadi said in a statement that Zarqawi's group "should not get involved in minor fights that only serve the occupation."

The statement, distributed at Ramadi mosques, said that voting by Sunnis would "answer the Iraqi and American politicians who claim that the resistance has no political agenda."

It was not clear whether the statement represented the views of the groups' disparate leaders.

In Taji, just north of Baghdad, an insurgent leader who served in Hussein's intelligence services oversaw guerrillas providing security at polling places on the eve of the referendum. As a reporter watched, he chastised a fellow insurgent for bombing another site.

In the far west, tribal fighters in heavily Sunni Anbar province deployed to protect some polling centers. Men holding AK-47 assault rifles took up posts at some sites in Fallujah. In Ramadi, however, local tribes reneged on a pledge to protect the polls, telling authorities that Zarqawi's group had posted a death threat on the gate of the home of a tribal sheik, said Khidhir Mohammed, head of the Anbar Provincial Council.

Iraqi military forces mobilized, under U.S. guard, to get ballots out to voting centers. For seven hours Friday, the Iraqi army delivered ballots to seven polling sites in the predominantly Sunni Arab town of Ishaqi and surrounding villages in Salahuddin province.

Two black Apache attack helicopters circled over the convoy, which consisted of three gun trucks and a transport truck filled with the election materials packed in cardboard boxes. The Iraqis were trailed by five armored Humvees with soldiers from the U.S. Army's 5th Battalion, 7th Cavalry Regiment. "We're not playing," said Lt. Col. Jody L. Petery, the battalion commander.

Three of the seven polling sites, all located at elementary schools, had been bombed Wednesday night, hours before Iraqi police were to move into the schools to provide round-the-clock security for the referendum. U.S. officers believed the bombers were tipped off by members of the Iraqi security forces.

The bombs destroyed desks and chairs and left tangles of wire that still had not been cleared Friday. But the U.S. military and Iraqi election officials said the polling sites were functional.

At least two polling sites in Balad, a predominantly Shiite city 20 miles south of Ishaqi, were hit by mortar attacks in the past week. One was moved to another location after a mortar shell landed on the roof.

U.S. and Iraqi commanders said they were uncertain who was behind the attacks.

Strict security measures went into effect across Iraq at the end of the week, including a curfew in Baghdad that began at 10 p.m. Thursday.

Carrying weapons in public was also banned for the days around the vote, and beginning Friday, the country's borders were sealed, Baghdad International Airport was closed and travel between provinces was prohibited. Most of the restrictions were to last until at least Sunday. No private vehicles will be allowed on the streets on voting day.

Unlike in January's parliamentary elections, when most Sunni Arabs stayed home and insurgents threatened Iraqis with beheadings if they voted, Sunnis have said they will participate in Saturday's referendum. A Western official in Baghdad said Friday he expected Sunni turnout to be strong, depending on security.

To defeat the proposal, opponents need a two-thirds "no" vote in at least three of Iraq's 18 provinces. But the solid support of Iraq's Shiite majority and Kurdish minority is expected to carry the referendum. State television broadcast a statement by Grand Ayatollah Ali Sistani, the most influential Shiite cleric in Iraq, recommending Shiites vote yes.

Fainaru reported from Ishaqi. Correspondents Jackie Spinner in Bakhan and Jonathan Finer in Najaf and special correspondents Naseer Nouri in Najaf and Omar Fekeiki, Bassam Sebti and Saad Sarhan in Baghdad contributed to this report.