BAGHDAD, Iraq - Insurgents determined to derail this weekend's referendum carried out two attacks against Iraq's largest Sunni Arab political party today, after the group dropped its opposition to the draft constitution.

The militants exploded a roadside bomb just outside the Iraqi Islamic Party office in central Baghdad, then set fire the party's main office in Fallujah, police said. No injuries were immediately reported in the capital or in Fallujah, a city west of Baghdad that was decimated by a U.S. offensive against insurgents in 2004.

Sunnis denounced the party's head, Muhsin Abdel-Hamid, as a traitor during a demonstration against the constitution in Baghdad.

"Muhsin is a U.S. agent," about 150 protesters shouted, carrying banners with slogans such as "No to the constitution."

The rare attacks against the party by Sunni-led insurgents appeared aimed at punishing it for deciding to end its "no" campaign against the referendum after lawmakers agreed Wednesday to several amendments to the constitution designed to win Sunni support in Saturday's vote.

"This attack by insurgents against the Islamic Party was expected because of its new stand toward the referendum," Iraqi army Maj. Salman Abdul Yahid said after the Baghdad blast. "Insurgents had threatened to attack the group and its leaders to get revenge," he said in an interview with The Associated Press.

Alaa Makki, a senior party official in Baghdad, condemned the attack in the capital and said it won't stop the moderate group's efforts to "use the political process to fight terrorism and promote stability in Iraq."

On Thursday, Iraqi Islamic Party banners urging a "no" vote had been removed from where they hung near monuments such as the Grand Imam mosque.

Other Sunni Arab parties still oppose the charter. They fear it would leave them isolated in central and western Iraq while Kurds in the north and Shiites in the south capitalize on Iraq's oil wealth.

In another insurgent attack in Baghdad today, the Muslim day of worship in Iraq, a roadside bomb wounded four Iraqi civilians when it exploded near one of the many schools in the capital that U.S. soldiers are fortifying with concrete barriers and barbed wire so they can be used as polling stations in Saturday's vote, said police 1st Lt. Mua'taz Saladin.

As police removed bloodstained shoes and shattered glass from damaged cars at the scene, one of the U.S. soldiers working there remained defiant. "This won't affect anything planned for tomorrow. The election will go off without a hitch," Lt. David Forbes said in an interview with an Associated Press Television News.

In Kirkuk, 180 miles north of Baghdad, a car bomb exploded near a Kurdistan Democratic Party office, wounding five civilians, said police Brig. Sarhad Qadir.

On Wednesday night, Iraq's National Assembly endorsed last-minute changes to the draft constitution worked out by Shiite, Kurdish and Sunni powerbrokers that will allow a new parliament scheduled to be elected in December to adopt amendments to the constitution.

The draft constitution is expected to pass in Saturday's referendum.