On Thursday - two days ahead of the vote - a nationwide night-time curfew will begin and no one will be able to carry weapons in public, even licensed ones.
"We will protect those who say yes and those who say no," Interior Minister Bayan Jabr said in Baghdad.
"We have countermeasures against all terrorist actions, and you will see tens of thousands of Iraqi security forces deployed in Baghdad and the provinces."
On Friday evening police will bar travel between provinces. International borders, airports and ports will be closed, but Jabr did not say when that step would begin.
"We will be looking out for mortars, suicide bombers with explosive belts, and car bombs," he said. "We will take every effort to provide protection."
He acknowledged problems with security in Iraq's western province of Anbar, a focus of the fighting. In Anbar's capital, Ramadi, only 1000 of the city's 6500-member police force were willing to come to work, Jabr said.
He said help from powerful local tribes was needed to protect polling stations, and that the Iraqi military would have to be responsible for security.
The influcence of local tribes is
needed in the Anbar province
The referendum has divided Iraqis, with leaders of the Shia Muslims and Kurds supporting the constitution while Sunni Arabs oppose it, sayng it will fragment Iraq.
Sunnis can defeat it if they garner a two-thirds no vote in any three of Iraq's 18 provinces.
Arab League role
A delegation from the Arab League arrived in Iraq on Saturday to lay the groundwork for an Iraqi reconciliation conference it hopes to hold after the vote. It was the first time the pan-Arab organisation has tried to take a direct role in Iraq since the 2003 US-led invasion.
"The situation is so tense there is a threat looming in the air about civil war that could erupt at any moment, although some people would say that it is already there," Arab League Secretary-General Amr Mussa said.
Arab League Secretary-General
Amr Mussa: The situation is tense
But in a sign of the bitter divisions, the league has received a cold reception from some Shia leaders in the government, resentful over perceived Arab League inaction over Saddam Hussein's government and what they see as the league's bias towards Iraq's Sunni Arabs.
Meanwhile, all sides were pushing their campaigns for and against the constitution. Leaders of the top Sunni political factions met in Baghdad to plan how to rally their followers in the no vote, with most of them pushing aside calls for Sunnis to boycott.
"The Sunni Alliance during its conference today called the Iraqis to reject the constitution - but not for boycotting the referendum," Mohammed Bishar al-Faydi, of the Association of Muslim Scholars, told Aljazeera. "It is a dangerous draft that only helps the interests of imperialism on our soil.
"This constitution is being imposed by force in a dictatorial way"
Mohammed Bishar al-Faydi,
Association of Muslim Scholars
"This constitution is being imposed by force in a dictatorial way."
In Tikrit, near Saddam Hussein's hometown, the Iraqi Islamic Party passed out copies of the constitution to worshippers at a mosque and urged them to go to the polls to reject it.
A strong Sunni turnout is key for defeating the constitution. Sunnis have enough population in four provinces to have a chance of getting a two-thirds "no" vote - but in several of those provinces there are significant Shia or Kurdish populations likely to be strongly motivated to vote "yes", so Sunni leaders must drum out their voters to outweigh them.
In the media
Government media was also pushing for voters to turn out. The state-owned Sabah newspaper ran a contest promising prizes up to $5000 for anyone who could answer a series of questions about the text of the constitution to prove they had read it.
Al-Iraqiya television ran call-ins and talk shows about the referendum, with people from all sides participating.
"I will vote yes to constitution because it's a guarantee
of our rights and an end to dictatorship," said Fadil Abbas, one man in a street interview.