BAGHDAD, Iraq - U.S. and Iraqi forces stepped up security across Iraq today in an effort to reduce insurgent attacks aimed at wrecking this weekend's constitutional referendum.

One day after Iraqi lawmakers approved a set of last-minute amendments to the constitution, cities were unusually quiet as a four-day national holiday began. Wednesday's deal sealed a compromise designed to win minority Sunni Arab support for the charter.

Government offices and schools were closed ahead of Saturday's vote, and a 6 p.m. to 6 a.m. curfew went into effect today. On Friday the country's borders will be closed and all travel among its provinces halted.

Working under cover of darkness, U.S. and Iraqi forces erected concrete barriers topped with concertina wire in front of polling places, to protect them from insurgent bombs.

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.

In the last 18 days, at least 438 people have been killed by militant violence as the insurgents try to scare voters away from the polls Saturday. Most of the fatalities have been caused by suicide car bombs, roadside bombs and drive-by shootings. The bodies of other Iraqis who had been kidnapped have been found in isolated areas.

Iraqis watching state-owned Al-Iraqiya television on Wednesday night saw legislators in the National Assembly approve a set of last-minute amendments to the constitution without a vote, sealing a compromise designed to win Sunni support and boost chances for the charter's approval in Saturday's referendum.

At least one major Sunni Arab party, the Iraqi Islamic Party, said it will now support the draft at the polls. But some other Sunni parties rejected the amendments and said they would still campaign for a "no" vote.

Iraq's most powerful Shiite cleric, Grand Ayatollah Ali al-Sistani, also weighed in, ordering Shiites to vote "yes" in the referendum, one of his aides, Faisal Thbub, said. It was the most direct show of support for the charter by al-Sistani, whose call brought out huge numbers of voters to back Shiite parties in January elections.

The most significant change is the introduction of a mechanism allowing Sunni Arabs to try to make more substantive changes in the constitution later, after a new parliament is elected in December. Sunnis want to weaken the considerable autonomous powers the Shiite and Kurdish mini-states would have under the constitution. But there's no guarantee they will succeed: They will still likely face strong opposition from majority Shiites and Kurds in the new parliament.

The amendments also made some key symbolic concessions to Sunni Arabs, starting with the first article underlining that Iraq will be a single nation with its unity guaranteed - a nod to fears among the disaffected minority that the draft as it stood would fragment the country.