Insurgents sabotaged power lines, knocking out electricity across Baghdad area Friday and plunging the capital into darkness on the eve of a landmark vote on a constitution aimed at defining democracy in a nation once ruled by Saddam Hussein (search).

For most of the day, Iraqis were hunkered down in their homes, with the streets of the Iraqi capital almost empty hours before a 10 p.m. curfew and the country sealed off from the outside world as borders and airports were closed for Saturday's referendum.

The approximately 140-article charter — hammered out after months of bitter negotiations — is supported by a Shiite-Kurdish majority but has split Sunni Arab (search) ranks after last-minute amendments designed to win support among the disaffected minority.

In Friday sermons across the nation, the message from Shiite pulpits was an unequivocal "yes," but it was not so clear-cut in Sunni Arab mosques — varying from "yes," "no" and "vote your conscience."

Insurgents, meanwhile, detonated a bomb outside the Sunni Islamic Party's office in central Baghdad, then set fire to the party's main office in Fallujah. Nobody was injured in what were apparently a symbolic attacks against that group's recent decision to support the charter.

Saturday's referendum, a key stop in Iraq's passage to democratic rule that the U.S. hopes will pave the way for withdrawing foreign troops, takes place as American and Iraqi forces battle an enduring Sunni-led insurgency in Baghdad and areas to the west and north.

"Besides Allah, we need this constitution to protect us," said Rajha Abdul-Jabar, a 49-year-old Sunni Arab mother of five married to a Kurdish dentist. "I, my husband and our children will go and vote yes tomorrow," she said in the small convenience store she runs.

Kurds, a sizable minority that is mainly Sunni, fully support the charter.

Jameel Safar, a 30-year-old Kurd in Baghdad, said the charter will safeguard Iraq's unity, but later added: "The Kurds are entitled to everything. We have a right to our own nation like everyone else."

Although there has been a lull so far this month in major insurgent attacks in Baghdad, the U.S. military has warned of an upsurge in violence to coincide with the vote.

Mahmoud al-Saaedi, an Electricity Ministry spokesman, said power lines were sabotaged between the northern towns of Kirkuk and Beiji leading to the Baghdad region. He did not specify how insurgents damaged the lines, but militants in the past have used bombs to hit infrastructure.

The lights went out soon after sundown, when Muslims break their daily fast during the holy month of Ramadan (search), and power was still off more than two hours later.

Baghdad's skyline was black except for pinpoints of light from private generators (search). The blackout appeared to have affected much of Baghdad province, an area of 2,250 square miles.

Tens of thousands of Iraqi army troops and policemen, meanwhile, formed security rings around the nation's estimated 6,000 polling stations and set up checkpoints on highways and inside cities.

The capital's usually congested streets were virtually deserted by late afternoon. Most shops did not open at all. Those that did closed early. Lines of cars a mile long waiting to fill up at gas stations provided one of the few signs of normalcy.

Ratification of the constitution requires approval by a majority of voters nationwide.

However, if two-thirds of voters in any three of Iraq's 18 provinces vote "no," the constitution will be defeated and Sunni Arab opponents have a chance of swinging the ballot in four volatile provinces — Anbar, Nineveh, Salahuddin and Diyala.

In Baghdad, security was particularly tight in flashpoint areas where insurgents are active, like the Sunni Azamiyah district in the north and Dora and Ghazaliya to the west.

Iraq's Kurdish President Jalal Talabani (search) and Shiite Prime Minister Ibrahim al-Jaafari (search) appeared on television late Friday to make last-minute appeals for a "yes" vote.

Talabani also called on the Sunni Arab insurgents to lay down their arms and join the political process. "Our dear brothers, the way to achieve your legitimate demands and rights is through political struggle ... and not terrorism or violence or boycotting," he said.

Most of Iraq's Shiites, about 60 percent of an estimated 27 million population, were expected to approve the charter, especially after Shiite spiritual leader Grand Ayatollah Ali al-Sistani (search) called on followers to do so.

In an effort to familiarize voters with the draft, local TV stations aired readings of amendments adopted this week, too late to be included in the U.N.-printed text distributed to Iraqis.

Those amendments persuaded the Iraqi Islamic Party, the country's largest Sunni Arab political group, to drop its opposition to the draft and call on its supporters to vote in favor of the constitution. The move split Sunni Arab ranks and boosted the chances for the charter's passage.

The minority, which had been dominant under Saddam, opposes a federalist system enshrined in the constitution that will let Shiites and Kurds form mini-states in the south and north. The draft was passed despite Sunni objections, but the issue remains.

In Tikrit, Saddam's hometown north of Baghdad, Sheik Rasheed Yousif al-Khishman exhorted worshippers at the al-Raheem mosque to reject the charter, saying the draft was an "infidel constitution written by foreign hands."

In the nearby town of Samarra — another bastion of Sunni militancy — Sheik Adil Mahmoud of the influential Sunni Association of Muslims Scholars delivered a more tempered sermon. "I will go to the polls and vote 'no,' but I leave the choice to you," he said.

A similarly moderate message came from Sheik Mahmoud al-Sumadai of Baghdad's Sunni Um al-Qura mosque. Every Sunni Arab should exercise his own judgment when voting Saturday, he said.

In the Baghdad district of Azamiyah, several hundred demonstrators carrying banners calling for a "no" vote and branding Islamic Party leader Mohsen Abdul-Hamid "a traitor" marched at the area's Grand Imam mosque, Iraq's main Sunni Arab religious center.

Before dawn, an assailant tossed a grenade at the house of the mosque's imam, pro-Islamic Party sheik Muayad al-Azami. Nobody was hurt in the explosion. The night before, his son was threatened by Sunni opponents during prayers, al-Azami said.

Most Sunni Arabs see the draft as a recipe for the eventual breakup of Iraq, saying it gives provincial governments too much control over natural resources, including oil, and does not sufficiently assert the country's Arab identity. The amendments endorsed by the Shiite-Kurdish dominated parliament on Wednesday addressed some Sunni Arab concerns, but not the key ones on federalism and Arab links.

If adopted, the draft charter will provide the basis for a general election in two months' time for a full-term parliament. If rejected, the December election will still take place but it will produce another interim chamber that will again attempt to draft a constitution.