Three offices of an Iraqi Sunni party which dropped its opposition to the new constitution have been attacked, a day before a referendum on the text.
The party changed its position on the charter earlier this week
In Baghdad, a bomb exploded outside the office of the Iraqi Islamic Party.
Gunmen set fire to the party's office in the western city of Falluja, and ransacked its office in Baiji, north of Baghdad. No injuries were reported.
Many Sunni parties oppose the text, and have called on Iraqis to boycott the poll or vote "No".
CONSTITUTION'S KEY POINTS
Iraq to be federal, parliamentary democracy
Official languages to be Arabic and Kurdish
Official religion to be Islam but religious freedoms guaranteed
Equal rights for all
Elections every four years
Security is being increased ahead of the referendum. Iraq's borders have been closed and private cars will be banned from Baghdad's roads for the vote on Saturday.
A new night-time curfew was imposed from Thursday night and a four-day public holiday is under way across the country, shutting government offices and schools.
Iraqi army Maj Salman Abdul Yahid said the attack on the Islamic Party offices "was expected because of its new stand toward the referendum".
Alaa Makki, a senior party official, condemned the Baghdad attack, saying the party would "use the political process to fight terrorism and promote stability in Iraq", the Associated Press news agency reported.
Earlier this week, the Islamic Party said it would encourage Sunnis to support the constitution after Shia and Kurdish political leaders agreed to consider further revisions after elections in December.
Sunni leaders fear the current proposals may lead the country to split, with a Kurdish north and Shia south, depriving Sunni Arabs of access to the country's oil resources.
The BBC's Richard Galpin in Baghdad says the controversy surrounding the draft constitution has made campaigning much more adversarial than expected.
It has been easier for the "Yes" camp who dominate government to get their message across on state-controlled media than for the Sunni minority, our correspondent adds.
Some 15.5 million voters are expected to walk to about 6,000 polling stations to register their view on the charter.
The election commission says it has managed to set up polling stations even in the regions worst-affected by violence, including the vast Anbar province, seen as the heartland of the insurgency.
If voters in three of Iraq's 18 provinces muster a two-thirds majority against the constitution, it will fail.
The Sunnis are dominant in four provinces and so therefore effectively hold a power of veto if they turn out in large numbers to vote against it.