Iraqi and U.S. forces have stepped up security across Iraq to stop insurgent attacks aimed at derailing Saturday's historic constitutional referendum.

A four-day public holiday has been declared and many streets in the country's major cities are quiet, with schools and offices closed.

In Baghdad, the usually chaotic traffic was scarce. Many shops didn't bother to open and others shuttered ahead of the nightly curfew. The large army and police presence, combined with the scarcity of people and vehicles, gave the city a disquieting calm.

Iraq's external and internal borders will be sealed Friday, with travel banned between provinces.

Baghdad's International Airport will be shut Friday and Saturday, while civilian vehicles are banned for two days starting Friday evening and a nationwide 10 p.m. curfew remains in force until Sunday.

Concrete and barbed wire barriers are being erected around polling stations in order to stop suicide car bombers and attacks by insurgents, who have left nearly 450 people during the past 18 days.

Reporting from Baghdad, CTV's Lisa LaFlamme said the chances of the referendum "diffusing the armed conflict" were "almost non existent."

"But this country has already proven that the threat of violence is no deterrent for voting," she told CTV News.

Meanwhile, thousands of Iraqi detainees in US and Iraqi-run jails have begun voting on the draft constitution.

It is not known whether former Iraqi leader, Saddam Hussein -- whose trial for crimes against humanity begins on October 19 -- will be allowed to vote.

Even with so few people on the streets, sharp divisions over the referendum were visible across Baghdad.

Hundreds of posters and banners urging a "yes" vote were plastered on virtually every wall and shop window in the Shiite district of Kazimiyah.

Earlier Thursday, Iraq's top Shiite cleric, Grand Ayatollah Ali al-Sistani, ordered his supporters to approve the constitution.

However, not a single referendum poster was visible in the Sunni district of Azamiyah, just across the Tigris River.

A banner by the Sunni Arab Iraqi Islamic Party urging a "no" vote was removed from where it hung a day earlier outside Azamiyah's Grand Imam mosque.

In the so-called 'Triangle of Death', a mainly Sunni area known for kidnappings and killings, there were no signs of posters either.

Iraqi troops searched cars under the watchful eyes of comrades manning machine-gun positions and U.S. helicopters hovered over the area.

U.S. military spokesman Major General Rick Lynch provided an upbeat assessment of the security situation ahead of the vote, arguing that the insurgent danger was far less than on the eve of the January 30 parliamentary election.

But he says he still expects a referendum spike in attacks.

"The insurgents have declared war on democracy and they're going to conduct horrific acts of violence," he told The Associated Press.

If the charter is adopted, a general election will be held in two months to elect a full-term parliament. If it fails, an election will still be held in December but only for another interim chamber that will try again to draft a charter.

The constitution will fail if it falls short of a simple majority or is rejected by two-thirds of voters in three of Iraq's 18 provinces.

U.S. President George Bush sought to rally U.S. troops in Iraq, saying "the enemy understands that a free Iraq would be a blow to their vision.

"We put in motion something that can't be stopped, and that is the march of freedom," he told soldiers based in Saddam's hometown of Tikrit in a video conference.

There are now 156,000 U.S. troops in Iraq, a total that has been rising in recent weeks as the 101st Airborne returns, along with lead elements of the 3rd Corps Support Command.

Security at the estimated 6,000 polling stations will be the responsibility of Iraqi police, who will be aided by Iraqi soldiers and coalition troops.

The last-minute amendments to the draft constitution adopted Wednesday were designed to win Sunni Arab support but did not meet all their demands -- primarily for a clear assertion of Iraq's Arab identity and a reduction of wide powers accorded to provincial governments that Sunnis say could lead to Iraq's breakup.